Astrophysicist

Category: What’s Up

What’s in store for 2024

Happy New Year! I spoke with Glen Hunt on BBC Radio Lancashire this morning about what the Universe (and our exploration of it) might have in store for us this year. It certainly looks like it will be an exciting year for space exploration, and there are many potential discoveries that could be on the cards. Here are some things I’m particularly looking forward to.

The Sky in January

The first thing to look for is the planet Jupiter high in the sky for most of the night. Look towards the South if you are in the northern hemisphere, look to the North if you are south of the equator. Jupiter will be bright, and pretty unmistakable in the early evening.  Look with binoculars, if you have them, to see its moons (and cloud bands, if you’re lucky enough to have access to a telescope).

The ringed planet Saturn is also visible this month, but low in the South-West and fainter. Look with binoculars or a telescope for a view of the impressive ring system.  Saturn sets before 9pm and gets earlier as the month goes on, so catch it early if you can.  There is also a crescent moon close by on the 14th, with both objects low in the South-Western sky.

A sky map showing the locations of some highlights of the January sky.
The sky as seen from North-West England at 6.30pm on January 3rd. Jupiter is high in the South, Saturn is over to the South-West, and the winter favourite constellation of Orion is rising in the East. Image: Stellarium / the author.

If you’re up early, have a look for Venus (easy to spot) and Mercury (quite a lot more difficult!) low in the eastern sky before dawn.  Use binoculars for a better chance of spotting the elusive Mercury, BUT DON’T LOOK AT THE SUN with them!

One of the best meteor showers, but one of the least well-observed (thanks to the weather!), is the Quadrantid meteor shower peaking on January 4th. Unlike most meteor showers which originate from comet debris, this one is due to asteroid 2003 EH1 (possibly a dead comet, possibly a rock comet). This asteroid has an orbit around the Sun lasting 5.5 years, only taking it as far out as Saturn, unlike the majority of comets which spend most of their time beyond Pluto.   Although the body responsible was only discovered in 2003, the meteor shower has been known about since 1825. The radiant, the position the meteors appear to all come from on the sky, was in the now-defunct constellation of Quadrans Muralis (hence the name) – this location is now part of the constellation of Bootes the Herdsman today. 

The Quadrantids can be a very strong shower, with peak rates of more than 100 meteors per hour, but the peak is very short-lived and this year is hampered by daylight during predicted peak hours here in the UK. You have a better chance of seeing good activity from the Americas or Asia this year. Your best chance to view is in the early hours when the radiant is high, after 2am.  Sadly the Moon all rises just after midnight and is half illuminated, making viewing fainter meteors more difficult.  The short predicted peak of activity is between 0900 and 1500 GMT on January 4th.  

The sky looking East at 2.22am on January 4th 2024 showing the constellation Bootes, the location of the radiant for the Quadrantid meteor shower. Image: Stellarium / the author.

The sky later in the year

There will be the usual changing of the constellations as the seasons roll around. Orion is a winter favourite with lots of bright stars and a spectacular star forming region below Orion’s belt that is worth a look with binoculars or a telescope.

There are also a series of meteor showers that happen on the same dates each year as we pass through the debris left behind by comets or asteroids as they orbit the Sun. The ones to watch in 2024, aside from the Quadrantids in January, are the Perseids in mid-August and the Geminids in December. Other meteor showers will happen, but they are not predicted to be as active as these. For most details, keep an eye on the IMO’s Meteor Shower Calendar which is regularly updated.

And a couple of supermoons. While these are not particularly exciting from a scientific point of view, the full Moon is always impressive, and a full Moon at lunar perigee is bigger and brighter than usual, so will be worth a look if the skies are clear. Look up on September 18th and October 17th 2024. (And remember, the astronomical term for this is perigee-syzygy of the Earth-Moon-Sun system. Snappy!)

Spaceflight

2023 was an exciting year for spaceflight and solar system exploration missions, and 2024 is looking like it will be just as exciting. There’s certainly lots planned, both by space agencies like NASA and ESA, governments such as China, and the rapidly growing commercial sector.  Here are some things to look out for in 2024.

Europa Clipper is scheduled to launch in October, heading off to survey Jupiter’s moon Europa, an icy world that may support a sub-surface ocean.  There is a lot of interest in exploring the potential for habitability of Europa (you need liquid water, the right chemistry, and light/heat) and this probe will help with that by allowing scientists to determine the extent of any sub-surface liquid water and examine the geological processes at work.  It won’t arrive until 2030 though, but will complement ESA’s JUICE mission (launched in 2023) nicely when it does.

China plan to launch the next in their series of missions to the Moon with Chang’e 6 heading to the far side of the Moon to attempt the first sample return from the far side of the lunar surface (the side we never see from the Earth).  Likely launch in May.  We have lunar samples from the near side thanks to Apollo, but not from the far side, so comparing the two will tell us more about the history of the Moon.

June should see the maiden flight of the long-awaited Ariane 6, the European Space Agency’s replacement for the long-serving and highly successful Ariane 5 launch vehicle.  It will be able to carry a large payload to orbit, but could lose out commercially to SpaceX’s Starship once that starts flying successfully, and Blue Origin’s New Glenn heavy launcher expecting it’s maiden flight in August. Ariane rockets are expendable, whereas Starship and New Glenn are largely reusable which brings down the cost per launch substantially.

And of course the next Artemis mission taking humans around the Moon for the first time since the Apollo era, pencilled in for November 2024 (but, like all launches, this could slip).  Artemis 2 will be the first crewed flight of the Orion capsule, and see the first humans go beyond low Earth orbit since 1972, taking four astronauts around the Moon and back in what is likely to be a highly-watched mission.  Expect stunning photographs of the lunar surface!

As well as large programmes like Artemis, NASA have also commissioned a series of commercial companies to provide lunar landers for robotic and autonomous probes through the Commercial Lunar Payload Services (CLPS) programme.  The first of these missions, Peregrine 1, is due to launch on January 8th and will carry two rovers, several sensors, and a collection of time capsules. Several other launches are planned, expanding NASA’s existing work with outside commercial contractors that goes back decades.

We are also likely to see both another record year for satellites launched into low Earth orbit (making the risk of collisions ever more likely, and astronomy more challenging) and more new private operators beginning launch operations, including multiple commercial launches to the Moon, adding further competition to the market.

Biosignatures?

And finally (in Jodcast tradition), there is certainly a lot of interest in looking for evidence of biological processes in space, and it’s possible 2024 might be the year we first find solid evidence of biosignatures somewhere other than the Earth. This could be in samples from Mars, or it could come from further afield in the form of spectroscopic signatures from the atmospheres of exoplanets.

Since the 1990s we have discovered more than 5000 exoplanets around stars other than the Sun. The more we look, the more we find. Remember though that this is still challenging – we had the telescope for almost 400 years before we discovered the first solid evidence for extra-solar planets! And as is usually the case with new discoveries, we found the easier ones first.

Astronomers are using powerful telescopes with very sensitive spectrometers to split up the light coming from the exoplanets. This is challenging because a planet only reflects the light from its host star, rather than emitting its own light, so it is much fainter than the star and much harder to detect (which is why it took almost 400 years to find one!). But if you can do it, and split the light into its constituent colours to make a spectrum, you can then look for the signatures of chemicals in the planet’s atmosphere.

Some chemicals are associated with life, so those are the ones to look for. Many of these chemicals can also be produced by non-biological processes too, so finding a signature in the spectrum is only one part of the puzzle in many cases. But if we find one of them, it will certainly be tantalising.

There are also a lot of astronomers looking for signatures of life in the cosmos using a rather different method. Instead of looking for biosignatures, they are looking for technosignatures – evidence of advanced technology from signals that could not be produced in any natural way. This could be radio signals (like the radio and TV transmissions we have been sending into space for many decades now), or something more advanced such as a powerful beacon, or evidence of megastructures, or even evidence of some cataclysmic event.

It may sound like science fiction, but these are genuine research projects. The probability of finding either a bio- or technosignature may be low, but should we find evidence of life elsewhere in the cosmos, the implications really would be tremendous.

That’s not all folks

There are plenty more exciting events to look out for in 2024, from super moons to comets, results from previous missions, new images from JWST that will be as astonishing as ever, and many, many rocket launches. But it’s lunch time here, so I’ll leave it there for now. If you made it this far, thanks for reading! What are you looking forward to this year?

Fireworks? In October?

No, not Bonfire Night. We’re talking celestial firework displays! It’s October, and once again we’re coming up on the time for the annual Orionid meteor shower.

There’s been a trend over recent years of various parts of the media getting a bit hysterical about various astronomical phenomena, and in some cases hyping them up waaaaaay beyond any sensible justification and raising expectations to totally unrealistic levels. So if you’ve arrived here having heard stories about how spectacular the Orionids will be on October 21st, should you believe the hype? TLDR: no, but…

What is a meteor, anyway?

A meteor is actually quite mundane: a small piece of rock, generally smaller than a grain of rice that disintegrates as it flies through our upper atmosphere while travelling at many kilometres per second.

You can see meteors on any clear night of the year, all you have to do is find somewhere dark, look up, and be patient. These are called sporadic meteors and are just the detritus left over from the formation of the solar system, or debris from collisions between rocky bodies in the solar system. In the absence of something like a planet to crash into, they just float gently around in space not doing very much.

Usually you have to wait a while before you see a sporadic meteor, although because they are distributed randomly in space they don’t turn up a regular intervals and you may see a handful close together if you’re lucky.

Meteor showers, on the other hand, can be much more spectacular, and come round predictably at the same time each year. In the case of the Orionids, that time is October 21st – or thereabouts.

Origins of the Orionids

All meteor showers are the result of the Earth passing through regions of space with a higher than average concentration of these dust and rock particles. This happens because stuff gets left behind when comets (and some asteroids) go about their normal business on elliptical orbits around the Sun.

Comets are a bit like giant, dirty snowballs, containing large quantities of both ice, frozen gases, dust and rocks, and other volatile substances. We currently know of more than 3800 comets in the solar system, some of which we have actually visited giving us a much better idea of their chemical composition. Samples collected by the Stardust mission even finding the presence of the amino acid glycine, one of the fundamental building blocks of all life as we know it.

Comets spend most of their lives in the far reaches of the solar system where conditions are very cold indeed. Since comet orbits are elliptical, and centred on the Sun, their orbits also take them into the inner solar system for some of the time.

As a comet approaches the inner solar system it get closer to the Sun and so absorbs more solar radiation, heating the nucleus and causing some of the ice to sublimate – that is it turns directly from a solid ice into a gaseous vapour. As the ice turns to gas, the dust and rock particles embedded in it are released and float away, leaving a trail of debris behind the comet as it travels around the Sun.

 When the path of the Earth happens to cross one of these debris trails, we see an increase in meteors coming through our atmosphere.  This is the origin of a meteor shower, and explains why they are regular with predictable dates of activity.

The Orionid meteor shower is the result of debris left behind by Halley’s comet, one of the most famous comets in our solar system. The comet only returns to our skies once every 76 years (and is not due back until 2061), but the Earth travels through the debris trail each year, giving us the regular Orionid shower in October, and also the less well-known Eta-Aquariid shower in April/May.

Variations

A given meteor shower may not have the same level of activity, year to year. Some years a shower might be fairly unimpressive, with peak rates of only a few per hour. Other years we might have a much larger spike in activity and see rates of several hundred an hour. Rarely we might see rates of more than 1000 per hour – rarely seen meteor storm.

Imagine an aeroplane passing through the sky on a sunny day, leaving a contrail behind it in the sky. If you sit and watch that trail, it slowly expands, becomes less dense, and eventually disappears. Comet debris trails are a little like that (although not as easy to see, being made up of tiny dark particles of rock!).

As that trail ages, it gets less well-confined, the particles move apart slowly. Each time the comet comes round on its orbit, it deposits a fresh trail of denser debris along its orbital path. Add in the fact that the gravitational influence of the likes of Jupiter (and any other planet the comet comes relatively close to) can alter the trajectory of the comet, and you start to get a sense of why the number of meteors we see varies from year to year as the Earth passes though denser or less dense parts of the debris trail.

Very clever folks (such as the IMCCE meteoroids and meteors group) take observational data on the number of meteors observed each year (collected by seasoned observers – and you can help!) and build models of the debris trails for each meteor shower, and use those to make predictions about the number of meteor showers we are likely to see the following year. They are usually pretty accurate, but sometimes we see unexpected spikes in rates that were not possible to predict from past data alone – so it’s always worth a look.

What to expect in 2023

This year we are expecting good observing conditions on the peak night of October 21st, with the Moon at 48% illumination but setting before midnight.  The best time to observe will be after midnight when the Moon sets and the constellation of Orion will have risen – this is the location of the radiant of this shower, the location on the sky the meteors appear to come from, and what gives the shower its name.

Some showers are particularly spectacular with more than one a minute on average.  We’re not expecting that this year for this shower, with predictions of around 20 or so per hour.  However, any shower has the potential to be spectacular, so it’s always worth a look!

The best way to observe is to find somewhere away from street lighting, wrap up warm, and look up!  Obviously, you also need clear skies, but don’t worry if October 21st is cloudy as the streams of debris that cause most meteor showers are wide enough to provide activity over more than one night.  Catching meteors takes patience, but can be worth the effort, and can contribute to citizen science projects.

Clouded out? I expect a lot of us (here in the UK, anyway) will be, thanks to Storm Babet. You might have more luck in other parts of the world. The nice thing about the Orionids is that they are equatorial, meaning you can see them from both the northern and southern hemispheres.

If you do miss them, don’t despair. There are plenty more meteor showers in the calendar!

In December we will see the return of the Geminid meteor shower with a predicted peak of ~150/hour! Much more spectacular! The Moon will also be favourable as it will be very close to the Sun and not up during most of the night, aiding dark sky conditions which help you see the fainter meteors.

So, if you miss out this weekend, make a note in your calendar of December 14th, and make sure you remember to take a look!

Exploding stars

If you’ve looked at any astronomy-related news over the last few days, you’ll have spotted that a supernova has exploded in a nearby galaxy, and astronomers are getting excited about it. Supernovae are not uncommon (we discovered more than 2000 of them just last year), but some get astronomers more excited than others. Why is this one interesting? Read on.

We think of stars like the Sun as lasting forever. On human timescales, that’s ok since most of them have lifetimes of millions to billions of years – our own Sun has been shining for some five billion years, and probably has enough fuel to last another five billion or so. But they are transient things, like everything else in the Universe, just on timescales it is difficult for us mere mortals to grasp. Even astronomers struggle!

Stellar lifecycles

Stars are giant fusion reactors, fusing hydrogen in the core to make helium, and releasing energy in the process. It’s this energy release that powers the heat and light we get from the Sun, and what makes all the other stars shine. Stars continue this fusion process for as long as the conditions are right in the core. You need a lot of hydrogen under very high pressure and at very high temperatures. Eventually, the hydrogen runs out and a star starts to die.

The way a star ends its life depends very much on one parameter: its mass. The amount of stuff that makes up a star. It’s a simple parameter, but it determines an awful lot about a star’s properties, lifespan, colour, luminosity, end point, and much else. Most stars are mostly composed of hydrogen (~75%) and helium (~25%) with small amounts of other elements. Stars that have formed recently (in cosmic terms) have a greater proportion of these “other” elements, for reasons I’ll come to, but they are still mostly hydrogen and helium.

When a star runs out of hydrogen in the core, the fusion process slows down, reducing the amount of energy released per second. That energy, in the form of photons, keeps the star from collapsing under gravity for most of the star’s life. When the number of photons decreases, the star can no longer counteract the relentless pull of gravity, and it starts to contract. In science terminology, the star is no longer in hydrostatic equilibrium.

As the star begins to contract under gravity, the pressure in the core begins to rise, the temperature also goes up, and we get to the point where conditions in the core are extreme enough that the fusion of helium can begin. This process also releases energy (although not at the same rate) and stops the collapse for a time. There isn’t so much helium though, so the helium-fusing period of a star’s lifespan is relatively short. The shell of mainly hydrogen around this hotter core is now also hotter, and can start undergoing hydrogen fusion too. Again, it doesn’t last anywhere near as long as the original hydrogen burning phase – a phase we call the main sequence lifetime, and which makes up the longest part of any star’s lifetime.

For stars like the Sun, that’s almost it. The helium also runs out, the core starts to collapse again. As the outer layers expand and cool, the star becomes a red giant. The core continues to shrink, eventually becoming a hot, dense object known as a white dwarf. The outer layers eventually drift off to form a (poorly-named) planetary nebula, the white dwarf core slowly cools and fades away over billions of years.

But for stars greater than a certain mass (somewhere around eight times the mass of our own Sun) the end is much more dramatic. These stars continue the fusion process, with the core developing like the layers of an onion, with each progressive layer fusing heavier and heavier elements as they are produced by the layers above them, all the way across the periodic table, up to iron. Iron is interesting, as its the first element in the periodic table where you cannot release energy through fusing it to create a heavier element. To fuse iron, you have to add energy. There’s nowhere inside a star for that injection of energy to come from, so the fusion process stops.

If there’s no fusion in the core, there is nothing stopping the layers above collapsing under gravity. And that’s what happens. Catastrophically.

The outer layers start to fall as the source of photon pressure to keep them up diminishes, until the iron core collapses under its own weight*, resulting in energetic shockwaves that literally rip the star apart. This is what we call a core collapse supernova. The physics of this are fascinating, and the theoretical modelling has come a long way in recent years as computers have become more powerful. (Other types of supernova happen too, astronomers recently detected radio observations from a thermonuclear supernova for the first time, but that’s for another day.)

Now, supernovae are not rare events. As I mentioned at the top, we discovered over 2000 supernovae just in 2022, with many more than that identified as candidate supernovae. Many of these are discovered by automated surveys that scan the sky each night looking for new objects, or astronomical objects changing in brightness. But some are discovered by dedicated amateurs with good equipment and an excellent knowledge of the sky.

SN2023ixf

This supernova, SN2023ixf, was discovered by one well-known supernova hunter, Koichi Itagaki from Japan. He’s discovered quite a few over the years! It is located in one of the spiral arms of the galaxy M101, also known as the Pinwheel galaxy due to its appearance, and it is pretty gorgeous:

M101 or the Pinwheel Galaxy, as seen by Hubble. Credit: ESA/NASA/Hubble.

The galaxy is some 21 million light years away, so the light reaching us now (including the light from this explosion) left the galaxy 21 million years ago and has only just reached us. Looking out into space is looking back in time, we only ever see the Universe as it was in the past.

So, if we discover so many of them each year, why are astronomers excited about this one in particular? Well, for one simple reason: its proximity. While we discover a lot of supernovae, most galaxies are a lot further away than M101, so most supernovae are also much further away than this. The closer a supernova is, the more easily we can study them, and the more detail we can achieve with our observations. A nearby event means that our telescope have better resolution (our images have more detail in them) and we collect more photons (an event of the same brightness twice as far away will appear four times fainter).

This galaxy has also been host to several supernovae over the years. Since we’ve been watching, we’ve spotted at least five in M101, the most recent being SN2011fe discovered in (no surprise!) 2011.

What we know so far

SN2023ixf was first discovered on May 19th and is being observed by astronomers around the northern hemisphere, so what do we know so far?

It’s a core collapse supernova, having gone through the evolutionary stages described above. Estimates from early modelling suggest that the progenitor star might have been about 15 times more massive than the Sun before it exploded. It has been getting brighter, heading for a peak magnitude of around mag 10 – far too faint to be seen with the naked eye, you’ll need at least a 4-inch/20-cm aperture telescope (and some patience!) to spot it.

There are lots of images over on David Bishop’s excellent Bright Supernova catalog page for this object, and plenty of updates if you do a quick search. It will likely stay bright form some weeks, so (as Catherine Heymans said on 5 Live at 8.50am this morning), if you have a telescope gathering dust in the attic, you might want to drag it out and have a look.

Why are they important?

So what’s all the fuss? It may be close, but it’s just one of many hundreds of thousands – so what?

Supernovae are cosmologically important. That nuclear fusion process I described makes elements up to iron in the periodic table. To get anything heavier than that, you need even more extreme conditions than in the core of a massive star, and that’s where supernovae come in.

In a supernova explosion, the extreme conditions created in the shockwave and subsequent blast result in a rapid burst of nucleosynthesis, creating elements much heavier than iron, including many that are important to the creation of things like the computer you are using to read this!

All of those heavy elements, as well as the stuff up to iron created before the explosion itself, get thrown out into the surrounding gas that make up what astronomers call the interstellar medium. This process is called enrichment, adding heavy elements to a gas that is mostly made of hydrogen and helium.

Stars form from the interstellar medium. Over dense regions in the gas start to clump together, gravity increases and more material gets pulled in. Slowly, over hundreds of thousands to millions of years, the gas accumulates, the temperature goes up, the pressure in the centre increases, until you have a hot ball of gas where nuclear fusion can begin in the core – et voila, a star is born!

The gas around the core then goes on to form planets in many cases. If that gas cloud only contains hydrogen and helium, you only get gas giants forming. If the gas also contains heavy elements in sufficient quantities, produced in previous supernova explosions, then you get rocky planets forming too.

If you’ve ever heard Carl Sagan talk about us being made of star stuff, this is what he means. Much of the chemicals that make life possible, that make up the crust the Earth, that make up you and me, and our computers, were made inside massive stars and released into space in supernova explosions.

Don’t panic?

Our own galaxy, the Milky Way, is also forming stars, some of which are massive enough to go supernovae in this way. We actually haven’t had one in the Milky Way since 1604 so we’re rather overdue for one.

Some stars we know of are big enough to explode this way. Betelgeuse is a red giant that has got astronomers very excited in the last couple of years by behaving in a way that could signal its impending doom. It hasn’t exploded yet, but keep an eye on it.

If a supernova did explode in the Milky Way it would likely appear extremely bright due to its closeness. Astronomers would go bananas.

Supernovae are energetic explosions, generating blasts of neutrinos, as well as strong radiation across the electromagnetic spectrum – radio, infrared, visible, ultraviolet, X-rays, gamma rays. Any planets in the neighbourhood would get a good dose of radiation.

Don’t worry though, there are no stars in the vicinity of the Sun that are big enough to be a threat to life on Earth by going supernova. The supernova candidates are all far enough away to not be a threat. They would be pretty cool to watch though!

Edit: I just spoke to BBC Radio 5 Live about this supernova! Listen back (5.50pm): https://www.bbc.co.uk/sounds/schedules/bbc_radio_five_live

Edit: thanks to Jost Migenda for spotting my mistake!

Planetary spectacular

You may have heard Prof Lucie Green talking about the planetary conjunction on BBC Radio 4’s Today programme this morning – there were five planets lined up neatly in the early-morning sky this morning! Don’t worry if you didn’t see it today, or it you tried and had cloudy skies, you can still catch it over the next few days.  Here’s where to look, and what to look for.

Mercury, Venus, Mars, Jupiter and Saturn all appear in a line in the early morning sky.

Planetary alignment of late June 2022 – visible view at 4.15am BST – as seen from NW England.

First thing is, you’ll need to be up early!  The view above shows the sky at 4.15am.  The Sun rises at 4.43am from where I am, so you won’t see much after that as the sky will be too bright to see anything other than the Moon!  If you can drag yourself out of bed at that time, here’s what you will see.

Looking East, with a good horizon (ideally up a hill, but anywhere that you can avoid tress, hills or houses to your East) you should be able to see in order going up from the horizon: Mercury, Venus, the Moon, Mars, Jupiter and Saturn.  That’s quite a view!  The Moon is only 14% illuminated, so will appear as a nice crescent shape.  At magnitude -3.9, Venus will be the brightest of the set, less than 10 degrees above the horizon at 4.15am.  Mercury is the trickiest to spot, but will be between Venus and the glow of the pre-dawn Sun.  At magnitude -0.3, it will be a challenge to spot in the skyglow as by this time it is still only four degrees above the horizon.  If you have binoculars you will find it easier to catch, but be very careful NOT TO LOOK AT THE SUN!  Moving a little round towards the South, Mars is next.  At magnitude +0.5 it will still be easy to spot – you’re looking for something with a reddish/orange colour to it.  Moving up and further South again, you will find the next brightest of the set, Jupiter.  With a magnitude of -2.4, this planet is always hard to miss in the night sky.  If you have binoculars, have a look and see if you can spot the four largest Moons of Jupiter: Io, Europa, Ganymede and Callisto.  Further round, almost due South at this time, you will find the last of the set: Saturn.  At magnitude +0.6, Saturn is a little fainter than Mars, but yellow rather than red in colour.  If you have your binoculars handy, have a close look and see if you can spot the rings.  If you have good optics and a steady hand, you might just see them!

[Aside: If you look carefully, you will also note that Uranus makes an appearance in the lineup.  You are unlikely to spot this without a telescope though, as it has a magnitude of +6.  In good conditions and with good eyesight, you might spot this with the naked eye during darkness, but not in the early hours with the Sun brightening the sky.  Not far from Venus is the Pleiades cluster of stars – now that is worth a look with the binoculars as it’s always an impressive sight.]

Why are the planets in a line?” I hear you ask.  That’s a good question, and it comes down to perspective.  The planets are actually always in a line, it’s just that it only becomes obvious when you have a close alignment such as this.  The reason for this is because all of the planets orbit the Sun is a very similar plane – you can imagine the solar system sitting on a dinner plate with the Sun at the centre and all the planets moving in (almost) circular orbits around the surface of the plate.  If you imagine yourself as an ant sitting on the dinner plate, you would see the planets sitting on a circle around you.  How does this look to us?  Here’s the same view as above, but now with this plane drawn on:

The planets all lie close to the plane of the ecliptic on the sky.

Planetary alignment of late June 2022 – visible view but with the plane of the ecliptic added.

This plane is actually the projection of the path of the Sun around the sky as seen from Earth.  We’re orbiting the Sun of course, not the other way around, but from our perspective we see the Sun move across the sky relative to the background stars over one calendar year.  The path the Sun takes across the sky is called the ecliptic by astronomers.  We do like our jargon.

The orbits of the planets in the Solar System lie close to the ecliptic line, which is the apparent path of the Sun as seen from Earth projected out into the sky.

Planetary alignment of late June 2022 – visible view but with the ecliptic and orbits of the planets added.

The above view is the same, but now I’ve added the paths of the planets as well.  You can see that, as the planets orbit the Sun, their orbits never take them very far from the ecliptic.  That’s because of that dinner plate effect I talked about earlier.  The planets are all moving about close to the plane of the solar system, and so are we, so they appear to closely follow the path of the Sun on the sky.  It’s not exact because the planets all have slightly non-circular orbits, and their orbits are all very slightly tilted compared to that of the Earth, but the planets are essentially always in a rough line from our perspective.  Pretty cool, huh?

Finally, if you’re finding it annoying that the Sun makes Mercury so hard to spot, you’re not alone.  Many astronomers have rarely caught a glimpse of it!  Since Mercury never moves very far from the Sun, and it’s quite small and rocky so doesn’t reflect a lot of light, it can be challenging to observe.  The best solution to this problem?  Visit the Moon where you don’t have an atmosphere to contend with!  If you viewed the sky at the same date and time from the (far side) of the Moon, here’s what you would see:

With no atmosphere, the sky does not appear bright blue as it does on Earth, hiding the stars during the day. Instead, the stars are visible all the time, whether the Sun is in the sky or not.

The same planetary alignment, but viewed from the Moon where there is no atmosphere to hide Mercury!

This is the view at the same date and time, but from a location of 25°43’N 157°19’E on the Moon’s surface.  The Sun is in the sky, but because the Moon has no atmosphere to speak of, there is no scattering of the Sun’s light, and the sky does not appear bright blue.  Instead, all the stars are still visible, just as if it were night time.  The Earth is below the horizon from here, so it’s not in the sky right now from this location.

As visiting the Moon is (sadly) not an option for most of us any time soon, my advice is to choose a nice hill, pack yourself some sandwiches and a flask of your favourite beverage, and go for an early morning hike.  Or camp up there with an alarm clock.  Good luck!

All images made with Stellarium.

© 2024 Dr Megan Argo BEM

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