Spend a month with the Moon

Observe the Moon in December 2020 and use it as your guide to discover other wonderful sights in the night sky tonight.

Though the Moon completes an orbit of Earth in 27.3 days, it takes 29.5 to complete a cycle of phases due to our planet's motion around the Sun. Image Credit: iStock

What’s the first thing you look for in the night sky? Most of us head straight for the Moon, and with good reason, as many of us got started in astronomy by watching our nearest neighbour. It’s an easy target and even beginners can get joy from observing the Moon and learning its phases.

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While the Moon is certainly wonderful on its own, it can also be our guide to the night sky: a jumping-off point to places all around the Galaxy.

Make a habit of keeping an eye on it and you’ll get a feel for the lunar phases. Also, from night to night you’ll see it move about 13° eastward relative to the much more distant background stars.

That’s a little more than the width of your fist at arm’s length. Watch closely and you might even be able to see the Moon creep along, slowly and silently, over a single night.

The Moon may always keep the same face turned to us, but the terminator makes sure that face keeps changing. Image Credit: iStock
The changing phases of the Moon. Credit: iStock

As it goes, the Moon meets up with new stars, planets and other distant objects, pointing the way so we can learn the skies. Events like these are known as conjunctions.

Here we’ll follow the Moon through a whole lunar month and focus on naked-eye targets that are observable along the way.

A lunar month is a full lunar orbit, or a lunation, from one phase back to that same phase again, usually from new Moon back to new. It takes about 29 Earth days to happen.

The next new Moon falls on 14 December 2020, so we’ll start there. On any night of this lunar month, just look for the date below. Then head out, find the Moon and look up; it’s that simple!

Let’s begin our tour with the Moon as our guide. But first, some top tips for observing the Moon. You can find more info for beginners towards the bottom of this article.

And, for weekly lunar phases delivered direct to your email inbox, sign up the the BBC Sky at Night Magazine e-newsletter.

Venus and the Moon Michael Bate, Milton Keynes, Buckinghamshire, 28 January 2020 Equipment: Olympus EPL-7 mirrorless camera, Sky-Watcher HEQ-5 Pro mount
Venus and the Moon captured by Michael Bate, Milton Keynes, Buckinghamshire, 28 January 2020.

Top tips for observing the Moon

1 Watch as often as you can to get a feel for the Moon’s timing and motion.

2 You can see amazing detail with your naked eye; those dark splotches are ancient lava beds.

3 The best time for viewing the Moon is from a few days after its new phase to a few days after full, when it’s out of the Sun’s glare and up early.

4 Don’t be surprised to see the Moon in the daytime. Travelling all the way around Earth, it spends half its time on Earth’s daytime side. A last-quarter moonset against a blue morning sky is a stunning sight!

5 See which phases you like best. I love them all but prefer a couple of days on either side of first quarter.

A month with the Moon

Early phases: new to first quarter

During the first quarter of the lunar month, the Moon will appear close to Jupiter and Saturn on 17 December. Credit: Pete Lawrence
During the first quarter of the lunar month, the Moon will appear close to Jupiter and Saturn on 17 December. Credit: Pete Lawrence

14 December It’s a new Moon, a phase that occurs when the Moon is between Earth and the Sun, hidden in the Sun’s glare and invisible to us.

If you’re in a long, narrow stretch of the Southern Hemisphere, mostly open ocean and parts of Chile and Argentina, you’ll see the Moon slide across the Sun’s face and block it out in a solar eclipse!

But we don’t have eclipses every lunation because the Moon’s orbital tilt puts it just above or below the Sun, from our point of view.

15 & 16 December Still in the Sun’s glare, the Moon is tough to see, but on the 16th, you may be able to spot a thin waxing crescent just after sunset.

17 December As the Moon pops into the night sky, we see it alongside Jupiter and Saturn, which appear very close (the two planets approach a Great Conjunction on 21 December). In this tiny corner of the sky are the two biggest planets and 162 of our Solar System’s moons.

The Moon, Saturn and Jupiter as they will appear in the night sky from the southwest of England, looking towards the southwest on 17 December 2020 at 17:10 UTC. Credit: Stellarium.
The Moon, Saturn and Jupiter as they will appear in the night sky from the southwest of England, looking towards the southwest on 17 December 2020 at 17:10 UTC. Credit: Stellarium.

18 December See that dim glow on the Moon’s left-hand side? That’s Earthshine: sunlight that bounced off Earth, travelled to the Moon, and then bounced back to your eye.

As we start the prime, easy-viewing part of the lunar month we’ve left the giant planets behind. The Moon’s now in a long line with Altair (Alpha (α) Aquilae) – which is part of the Summer Triangle asterism – the brightest star in Aquila, high above it and toward the west. Fomalhaut (Alpha (α) Piscis Austrini) is toward the south.

19–21 December On its way to meet Mars, the Moon spends these nights crossing the seemingly empty ocean of sky that gives Fomalhaut its ‘loneliest star’ nickname. Fomalhaut is the southernmost first magnitude star visible in the Northern Hemisphere, so you may need to hunt for it above the horizon.

Middle phases: first quarter to full

As the Moon heads towards its full phase, it will be visible near Mars before meeting the Winter Hexagon. Credit: Pete Lawrence
As the Moon heads towards its full phase, it will be visible near Mars before meeting the Winter Hexagon. Credit: Pete Lawrence

22 December First quarter is when the Moon is off to Earth’s left, as seen from the Northern Hemisphere while facing the Sun. It trails after Earth in our orbit, and its right-hand half is illuminated.

23 December We made it to Mars; tonight the Moon points us right towards the Red Planet’s festive glow. Notice how the shadows of the hills and mountains on the Moon’s waxing gibbous disc stretch toward its darkened part; dawn breaks on more of its face. For more info, read our guide on how to observe Mars.

24–25 December Merry Christmas! Now a heavy, waxing gibbous, the Moon’s shadows are shrinking as the long Sun-Earth-Moon line keeps straightening.

With binoculars you may be able to see Uranus in the darkness. It’s above the Moon to the left at about the same distance as Mars on Christmas Eve, and midway between the two on Christmas night.

The Moon Waxing Gibbous, by William Wood.
A waxing gibbous Moon, photographed by William Wood.

26 December The 90%-lit Moon makes a triangle with the dipper-shaped Pleiades and V-shaped Hyades clusters. Reddish Aldebaran (Alpha (α) Tauri) at the top of the ‘V’ isn’t actually part of the Hyades.

27 December The Winter Hexagon is an asterism of six of the night’s brightest stars, in six constellations (read our guide on the difference between asterisms and constellations). It’s so enormous that it will take the Moon the next four nights to cross it.

Clockwise from the Moon, the stars are Aldebaran in Taurus, Rigel (Beta (β) Orionis) at the foot of Orion, Sirius (Alpha (α) Canis Majoris) in Canis Major, Procyon (Alpha (α) Canis Minoris) in Canis Minor, Pollux (Beta (β) Geminorum) in Gemini and the stunning yellow Capella (Alpha (α) Aurigae) in Auriga.

Betelgeuse and Rigel. Credit: iStock
Betelgeuse and Rigel in the Orion constellation. Credit: iStock

28 December Now nearly full, the Moon is about midway between the giant reddish star Betelgeuse, which forms one of Orion’s shoulders, and second magnitude Elnath (Beta (β) Tauri), one of the horns of Taurus, the Bull.

The Moon is very bright for the next couple of nights, but that glare adds a certain stark warmth to the night. Elnath is toward the Galactic anticentre. As you look out at that star, imagine your gaze drifting beyond the Milky Way and out into the depths of the Universe.

The Galactic centre is back towards the Sun, towards new Moon, nearly opposite where the Moon is now.

Full to last quarter

As the Moon ushers in the New Year, it moves away from the Winter Hexagon. Credit: Pete Lawrence
As the Moon ushers in the New Year, it moves away from the Winter Hexagon. Credit: Pete Lawrence

29 December Tonight is full Moon! Now we’re halfway through the lunar month and the Moon has moved behind Earth. Shadows are flat because the long Sun-Earth-Moon line is straight, running in that order.

Full Moons rise around sunset and set around sunrise. Towards the horizon, you might be able to find second magnitude Alhena (Gamma (γ) Geminorum), which forms the foot of Pollux, one of the Twins, while his and Castor’s heads are toward the east.

30 December Treat yourself to the sublime sight of a setting full Moon to start your day and then, after your part of Earth has turned away from the Sun, see the Moon rise again, now in a waning gibbous phase between Pollux and Alhena.

21 day old, Waning Gibbous Moon captured by Harvey Scoot from Essex in the UK.
21 day old, Waning Gibbous Moon captured by Harvey Scoot from Essex in the UK.

31 December – 1 January 2021 Happy New Year! I think we’re all glad to say goodbye to 2020, as the Moon says goodbye to the Winter Hexagon.

2 January Prime Moon-watching ends for now as the Moon rises too late for many of us. If you stay up, you’ll see it alongside Regulus (Alpha (α) Leonis), in the constellation of Leo. For tonight, the Moon is part of the Sickle asterism that makes up the Lion’s front end.

3-5 January The Moon doesn’t get higher than 20˚ above the horizon until around midnight or later now. It crosses under Leo’s belly as it goes, and then crosses into the constellation of Virgo, the Virgin.

Last quarter to new

In the last quarter of the lunar month, the Moon becomes a morning object. Credit: Pete Lawrence
In the last quarter of the lunar month, the Moon becomes a morning object. Credit: Pete Lawrence

6 January Last quarter is when the Moon appears to Earth’s right, as seen from the Northern Hemisphere while facing the Sun. It’s actually ahead of Earth now, leading us along in our orbit.

The last quarter Moon rises around midnight, so there’s not much to speak of tonight. This is when the Moon becomes a morning object.

7 January In the tiny hours we’ll see Spica (Alpha (α) Virginis) just to the Moon’s south and Arcturus (Alpha (α) Boötis) to the east. With Regulus from a couple of nights ago, the stars in this part of the sky are a sneak preview of spring.

Waning crescent Moon captuted by Paul Licorish from London, UK.
Waning crescent Moon captured by Paul Licorish from London, UK.

If you’re not used to early morning stargazing, take in the quiet and the emptiness around you. I love waning crescents; they’re backwards from what you’re probably used to; it’s a bit unsettling, but they always make me smile.

8–9 January Deeper into the morning we go. When the Moon finally does rise, it’s near Libra’s flamboyantly named stars Zubenelgenubi (Alpha (α) Librae) and Zubeneschamali (Beta (β) Librae): on one side on the 8th, and the other on the 9th.

In a few months, we’ll see these in the southern sky where they’re like a gateway to summer, with spring’s Corvus and Crater to their west, and summer’s Scorpius and Sagittarius to the east. See if you can glimpse some more Earthshine on these mornings.

A simulation of a thin crescent waning Moon near the star Antares, rising before the Sun on 10 January 2021. Credit: Pete Lawrence
A simulation of a thin crescent waning Moon near the star Antares, rising before the Sun on 10 January 2021. Credit: Pete Lawrence

10–11 January The Moon is very difficult to see, rising just a short time before the Sun does. It’s a thin crescent just to the east of reddish Antares (Alpha (α) Scorpii) on the 10th.

This is your last real chance to see the Moon until it comes out on the other side of the next new Moon. It’ll be tough, but the Moon will be west of Venus on the 11th.

12 January This lunation ends with the Moon nearly impossible to see, as it rises just before the Sun. New Moon is tomorrow, and we’ll start the cycle all over again. There’s no eclipse this time, though.

There’s magic in the skies every night, and if you’re just starting out, you can always count on the Moon to guide you through the stars and show you the way to some amazing things. We hope you enjoyed this month in the life of the Moon, and hope you’ll let us know how it goes!

Get in touch by emailing us at contactus@skyatnightmagazine.com.

For more tips, read on as The Sky at Night’s Pete Lawrence reveals his advice for first time Moongazers.

Observing the Moon for beginners

Words: Pete Lawrence

The Moon Waxing Gibbous, by William Wood.
A waxing gibbous Moon, photographed by William Wood.

The Moon’s an ideal object to start your observing odyssey because it is big, bright and covered with amazing detail.

Roughly one-quarter the size of the Earth, our close companion always displays the same face towards us, a result of tidal forces in the Earth-Moon system synchronising its rotation to match its orbital period.

At any one time, half the Moon’s globe is lit by the Sun. Throughout the course of an orbit we get to see different amounts of the lit hemisphere, giving rise to the Moon’s phases.

Lined up with the Sun, the Moon’s lit half points away from us, producing what’s known as a new Moon.

Slowly emerging from its new phase into the evening sky, the lunar crescent thickens from one day to the next.

The term ‘waxing’ is used to indicate this thickening phase. The waxing crescent leads into the first quarter Moon, appearing as an illuminated semi-circle, roughly a week after new.

Even with the naked eye, on a clear night the Moon is a beautiful spectacle to behold. Credit: iStock
Even with the naked eye, on a clear night the Moon is a beautiful spectacle to behold.
Credit: iStock

‘First quarter’ refers to the fact that the Moon has completed one-quarter of its orbit. The bulging phases after first quarter are known as waxing gibbous.

These increase in size until roughly two weeks after new, the Moon is on the opposite side of its orbit from the Sun and appears fully lit as a full Moon.

After this the phases reverse, and the illuminated part of the Moon begins to shrink or wane.

After passing through the waning gibbous phases, the Moon reaches the three-quarter point of its orbit, giving rise to the ‘last quarter’ phase.

The Moon takes the appearance of a semi-circle once again, although it’s the opposite half that is illuminated than that at first quarter.

After this, it takes approximately a week for the Moon to go through its waning crescent phases, visible in the early morning sky, before it once again becomes new and the whole cycle starts again.

The phases of the Moon. Credit: Yaorusheng / Getty Images
The phases of the Moon. Credit: Yaorusheng / Getty Images

Getting to know the Moon’s terminator

The dividing line between the lit and dark portions of the Moon’s surface is called the terminator. If you stood in the lit portion close to the terminator, the Sun would appear low and you’d be casting a dramatic shadow behind you.

As you travelled further into the lit portion, away from the terminator, the Sun would climb higher and your shadow would shorten.

The same effect occurs with the physical features on the lunar surface. Those which lie close to the terminator cast dramatic shadows which exaggerate their presence. Those further from the terminator show hardly any shadows and are harder to make out.

So ironically, the worst time to observe the Moon is when it’s full because then, hardly any feature shadows are visible.

Mare Crisium on the Terminator by Ian Peck, Londonderry, Northern Ireland.
Mare Crisium on the Terminator by Ian Peck, Londonderry, Northern Ireland.

The Moon’s orbit is elliptical, meaning its distance from Earth changes slightly over time. When closest it speeds up slightly and slows down when more distant.

This small variation is enough to give us a chance to see a little more around the Moon’s eastern and western edges.

The Moon’s orbit is also slightly inclined and this causes it to sometimes appear above the Earth’s orbital plane and sometimes below. This gives us an opportunity to peek over the top, and under the bottom, of the Moon over time.

Thanks to lunar libration we can observe slightly more than half of the Moon’s surface. Credit: Pete Lawrence
Thanks to lunar libration we can observe slightly more than half of the Moon’s surface. Credit: Pete Lawrence

Taken together, this rocking and rolling action, known as libration, allows us to see a total of 59% of the Moon’s globe, sometimes revealing tantalising features normally hidden from view.

The upshot of all this variation is that any view of the Moon is as exciting as those that have taken place before it. So the next time you get a clear night and the Moon’s up, don’t pass up the chance to take a look at this tantalising world.

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Scott Levine is a US naked-eye astronomy enthusiast based in New York’s Hudson Valley. Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night.