Stargazing: Pocket guide to the Moon

Updated

It's 50 years since humans first walked on the Moon. Can you point out where Neil Armstrong and Buzz Aldrin landed?

Join astronomer Fred Watson on a guided tour of the Apollo sites, stunning lava plains and craters that dot the lunar surface.

Put your headphones on and let's go. There's something to see almost every night if you know where to look.

If you are planning to go outside:

Use an online lunar phase calendar to tell you what the Moon will look like and when it rises above the horizon.

You'll get the best view of the Moon's features through binoculars or a small telescope. The Moon is very bright so while you won't damage your eyes, you may need to cover large binocular lenses with paper caps with smaller holes if there is too much moonlight.

The Seas and the 'man in the Moon'

The Moon takes 27 days, 7 hours, 43 minutes and 12 seconds to travel around the Earth — around about the same time it takes to spin once on its axis. This means we only ever see the one face and the same familiar markings.

Our side of the Moon is dotted with many greyish patches. Early sky watchers thought these were seas — or maria — but they're actually ancient lava flows.

In 1651, Italian astronomer Giovanni Battista Riccioli gave each of the 22 seas names ranging from Mare Anguis — the Serpent Sea — to Mare Vaporum or the Sea of Vapours.

A number of these seas are incorporated into a vast 'ocean' — the Ocean of Storms — which covers an area of 4 million square kilometres.

If you live in the northern hemisphere the Ocean of Storms is on the top left, whereas in the southern hemisphere it is on the bottom right.

Our side of the Moon is very different to the far side, which has a lot of craters and mountains, but very few seas.

If you squint at the grey patches on our side of the Moon, you may be able to make out a face. This 'face' looks very different depending on where you live.

The left eye in the face as it is seen from the south is the darkest — and most famous — of all the seas: Mare Tranquillitatis or the Sea of Tranquillity.

Hey, if you're reading this, you're seriously missing out

The interactive, audio-driven version of this moon guide is really something special. You really should check it out — tap through and choose the 'get started' button.

'One small step for man' on the Sea of Tranquillity

Astronauts Neil Armstrong and Buzz Aldrin made history in July 1969 when they became the first humans to step onto a world beyond our own.

They landed the Eagle lunar module on the top edge of the Sea of Tranquillity, as we see it from the southern hemisphere.

Apollo 11 landed towards the end of the waxing crescent phase — just before half of the Moon's face as seen from Earth is illuminated. The astronauts used the shadows cast by the sun during this phase to identify landmarks and help guide them to the surface.

The astronauts brought back three new minerals from the Sea of Tranquillity that were later discovered on Earth: armalcolite — named using the first letters of their last names; tranquillityite — named after the sea; and pyroxferroite.

Apollo 11 was the first of six missions to the Moon. The landing site of the last mission to the Moon Apollo 17 is directly opposite Apollo 11 where the Sea of Tranquillity meets the Sea of Serenity at the bottom of the Moon's left 'eye'.

While we can't see much detail of the Apollo sites from Earth, they can be seen by spacecraft that orbit the Moon. The Apollo 17 site is the messiest of all the sites with traces of footprints and a moon buggy still visible.

Did you know the Moon has mountain ranges?

Across the other side of the Sea of Serenity from the Apollo 17 landing site is a large mountain range: the Apennines.

It includes the Moon's tallest mountain, Mount Huygens, which is 5,500 metres high. While it is the tallest mountain, it is not the highest point on the Moon, which is found on the far side.

The mountain range was formed by an asteroid smashing into the Moon about 3.9 billion years ago to create the Sea of Rains — or the 'mouth' of the face as we see it.

The Apollo 15 mission landed in a valley at the foot of this mountain range.

Those 'rays' were made by a meteorite

While the seas look grey, the lighter parts of the Moon are craters and highlands. Two of the most prominent craters on our side of the Moon are Copernicus and Tycho.

Copernicus, which is named after the astronomer Nicolaus Copernicus, lies above the Sea of Rains in the Ocean of Storms. At 93km across, it is wider than all but five craters on Earth. But it is nothing compared the South Aitken Basin on the far side, which is a whopping 2,500km across.

Just above Copernicus, still in the Ocean of Storms, is where Apollo 12 and 14 landed.



Towards the top right of the Moon is a slightly smaller crater, Tycho — named after another 16th century astronomer Tycho Brahe. If you look carefully, you'll see bright rays stretching out from the crater. These rays were created by the material ejected as a meteorite slammed into the area.

One of those rays extends thousands of kilometres down to the Sea of Tranquillity.

Rock samples collected by the Apollo 16 mission, which landed in the highlands above the Sea of Tranquillity, indicate Tycho is only 108 million years old. That's very young for the Moon — most craters are around 4 billion years old.

The terminator: When to see features at their best

The Moon looks pretty when it's full, but you'll see more details during other phases when the sun lights up the seas, mountains and craters from an angle.

These features will be revealed by the terminator — the line between light and dark.

The Moon's phases are created by its position in relation to the Sun.

We don't see the Moon at all at the beginning of the cycle — known as the new moon — because the far side is facing the sun and our side is completely in shadow.

But a couple of days into the cycle a thin sliver of light (the waxing crescent moon) appears. You'll get the best view after sunset just before it sets below the western horizon.

In the northern hemisphere the terminator moves from the right to the left, in the southern hemisphere the line moves from the left to the right. It lights up the same features, it's just our perspectives that are different.

As the line sweeps across the Moon each night you'll see more seas: first the Sea of Crises, then the Sea of Fertility, the seas of Nectar and Tranquillity and then Serenity.

When the Moon is divided in half — known as a first quarter moon —you'll see craters around Tycho towards the top.

When it gets fuller or gibbous, look for the Apennine mountains, Sea of Rains, Ocean of Storms, Copernicus and craters on the right-hand margin.

After the Moon passes full the features start to disappear in the same order — and it rises much later in the night until, around 29 days later, it is back to the new moon.

Earthshine: The old moon in the arms of the new

You'll catch another stunning phenomenon as the Moon disappears, or reappears, as a slim crescent: Earthshine.

While the terminator lights up the edge of the Moon, the rest of the face appears to have a ghostly glow. This effect is caused by sunlight reflected back from the Earth onto the Moon.

Leonardo Da Vinci was the first to work out what caused it in the 16th Century.

Also known as the "Da Vinci glow" or "the old moon in the arms of the new", Earthshine is most noticeable about one to five days before or after a new moon, and is more prominent in spring.

You'll catch it just after sunset (waxing crescent) or just before dawn (waning crescent).

Moon illusion: Why does the moon look so big?

The Moon is about a quarter of the size of Earth and, completely by coincidence, appears the same size as the Sun in the sky. But when it is low down in the sky it looks bigger.

This is most noticeable during a full moon as it rises above the horizon at sunset and sinks below the horizon just before dawn.

But this is an optical illusion known as the moon illusion. Basically, our brains are rubbish at working out size and distance in space.

If you hold a pencil at arm's length when the Moon is low on the horizon, and then a little later when it is higher in the sky, you'll notice that it's the same size.

The Moon's orbit isn't perfectly round, so sometimes it's closer to us (perigee) and sometimes it is further away (apogee).

If a full moon coincides with perigee, we have what some people call a "supermoon".

In reality, we can't really tell the difference between the closest point (about 363,000 km) and the average distance (about 380,000 km).

So while a rising "supermoon" looks big, it's really not much larger than a normal moon, and what we see is largely due to the moon illusion.

About your guide

Professor Fred Watson is Australia's first astronomer-at-large, author and science communicator. Working at the Australian Astronomical Observatory, he is passionate about preserving dark sky spaces to everyone can enjoy the night sky.

Credits

Reporter, producer: Genelle Weule

Astronomer & narrator: Fred Watson

Audio producer: Joel Werner

Designer, producer: Ben Spraggon

Developer: Colin Gourlay

Additional development: Joshua Byrd

Editor: Cristen Tilley

With many thanks to our community of remarkable astrophotographers: Dylan O'Donnell, Geoff Wyatt, Luke Lopez-Planells, Angel Lopez-Sanchez, Shelley Lloyd

Additional photos: NASA

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Topics: astronomy-space

First posted