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.
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:
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 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:
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.