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Introducing the Moon
Introducing the Moon

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Moon was made way back when a large chunk of cheddar the size of Mars hit Earth. True? No! Hold on to your Double Gloucester as this myth, and others, are about to be disproved!

Sea of Serenity by Craig Heaton

Sea of Serenity by Craig Heaton

To state a fact: the Moon is always around somewhere in the sky at some time or other. It sounds obvious, but this concept is wrapped up in urban myth and provoked a great discussion among planetarium presenters recently. The myth in question has two parts, both of which are entirely wrong: firstly, the Moon comes out at night; and secondly, it's invisible during the day. No names will be mentioned here to protect the guilty, but I was told by a science teacher just the other day (which makes the following even worse to absorb), that the myth is taught in a particular primary school because the pupils would be uncomfortable with the truth. I'd suggest that maybe the wrong teacher is in charge of science.


Understanding the Moon's orbit around Earth and the corresponding way it is lit by the Sun, which leads to its phases, is not easy. However, with a bit of patience it is possible to appreciate why the Moon can be visible during the day and does not only 'come out' when it's dark at night.

The question really should be: why don't we notice the Moon during the day? The simple answer is that the sky is brighter during the day than at night, so the Moon is not as prominent. There are lots of other reasons why, such as the fact that evenings are the part of the day when we generally have more time to gaze at the sky, whereas we're all busy rushing around during the day.

So, yesterday, after one such busy day at work I took the time as night approached to do some stargazing and watched the full Moon rise over the trees in the field in front of our house. To the naked eye, this is when the Moon is most magnificent: a low, golden-tinged globe slowly being carried into the sky by the rotating Earth. It's when the Moon is low that it's possible to notice this movement the most and, if you have time, it's worth noting where it is an hour later — you may be surprised how far it has moved.

A couple of other things that are deserving of special attention are the Moon's color and its movement across the sky as it orbits Earth.

The Moon moves at a blistering Mach 3, which is three times the speed of sound at sea level. The stronger goldish-to-reddish color of the rising Moon, as opposed to the grey-white view when it's higher, is explained by the fact that when it's low to the horizon, light from the Moon gets filtered through Earth's atmosphere. All the particles that make this up scatter the blue light, which leaves mostly red light to reach us when we're watching the spectacle on the ground.

A Moving Moon

As for the Moon's own travels, try and find a star very close to its left side. An hour later the Moon will have passed over the star, which should now be sitting to the Moon's right. The Moon may not completely cover the star, sometimes the star will just graze its top or bottom. You'll have more success seeing this if the Moon is not full, as its light washes out most nearby stars.

The passing of the Moon in front of the star or planet, blocking it, is known as an occultation and these are listed in the handbook of the British Astronomical Association. If you've never seen a star being blinked out by the Moon, then you have an assignment — go out and see one! Even better is a planet: Venus or Saturn are my favorites. Saturn is especially good: with a scope you can watch as the rings are slowly covered by the Moon, dipping in and out of the lunar valleys before finally disappearing.


The Sun is the main object that lights the Moon, but Earth also has an effect. Our planet is over 3.5 times the diameter of the Moon, so we reflect more light onto its surface than the Moon gives us when it is full. This is called earthshine and it can be seen as a faint glow on the unlit part of the Moon when it is a thin crescent (before and after a new Moon).

Phases of the Moon

The word Moon is responsible for our word 'month': one mooneth (or thereabouts) was the measurement of the time it took for the Moon to complete one orbit of the Earth in relation to the Sun. Although it's given in schools as 28 days, the Moon's changing appearance (it's cycle of phases) takes just over 29.5 days. This is known as the Synodic Month. It represents the period from one full Moon to the next (or any other identical phase, for example half Moon to half Moon).

The Sun is always shining on one half of the Moon — how much of the lighted side we see depends on where the Moon is in orbit around Earth. The new Moon happens when the Moon sits between us and the Sun, and so the far side is lit (this is also the only time a solar eclipse can occur).

As the Moon moves around Earth and each day passes, we see more and more of its lighted side, a waxing (growing) evening crescent first, then half Moon, waxing gibbous and finally full Moon. At this point, the Moon sits on completely the opposite side of the sky to the Sun. Now everything reverses and the waning (shrinking) phases go through gibbous, half and waning morning crescent, finally back to new.

Observing the Moon

The Moon: it's big, round and bright. Anyone can discover its finer details, whether it's with the naked eye or binoculars.

Some astronomers seem to get a complex about the Moon. It's not that they're affected by it in werewolfish ways, but rather they develop a loathing for our large, rocky satellite. Why? Well, these usually friendly astronomers come to see it as a natural light polluter, washing away all the faint, small and fuzzy galaxies and nebulae they like to view. To them, the Moon is more of a nuisance than an object that's worthy of observing.

This is a real shame as the Moon has so much to offer. There's simply no truth in the assertion that when 'you've seen it once, you've seen it all' — with binoculars and small telescopes the appearance of the Moon can change dramatically from one hour to the next. Another reason it's so good is that it's easy to find. There's no star-hopping or fiddling with finderscopes, as the Moon quite plainly hangs about just waiting for you to look at it.

Magnify the view

The Moon is a stunning object to look at, but there are times when binoculars or a telescope are the only things that'll do it justice: for example, the first few days after new Moon through to just before full Moon. During this period, when the Moon is waxing, we see a sunlit, happy side and a contrasting unlit, spooky side.

The views of the bright side give us the names of the phases: crescent, half, gibbous and full. After full Moon the phases reverse as it starts waning; these are equally worth a look. However, the post-full phases are generally seen very late in the night, when most people prefer to sleep.

The zone between the light and dark sections of the Moon is known as the 'terminator', and this is the place to concentrate on for the most stunning lunar views. It's along the waxing Moon's terminator that, if you were standing on the lunar surface, the Sun would be rising. The low light hits its mountains, craters, valleys, crinkly ridges, rilles, escarpments and all manner of other volcanic and impact features, casting dramatic shadows across the stark landscape. The view is further enhanced by largely flat, dark areas of solidified lava known as the lunar seas, over which shadows can stretch for tens of kilometres. All of this gradually changes as the Moon spins on its axis, but even at this slow rate you will be able to see hour-by-hour movement.

That the Moon spins on its axis may seem strange, as we know the same side always faces Earth. We are actually able to see 59 per cent of its surface as the Moon 'wobbles' up and down and from left to right, an effect known as libration.

We only see the one face because a long time ago the molten material inside the Moon caused it to become tidally locked to Earth. This 'synchronous rotation' means that the Moon spins once on its axis in exactly the same time it takes to orbit Earth. You can get an idea of how this works if you imagine yourself observing from the Sun. Over the course of a month you would see the Moon spin once.

Of course, leaving Earth takes a bit of mastering, but once you can imagine it, understanding the Universe becomes a breeze.

Latin Lingo

When you look at a map of the Moon, you'll notice that its physical features all have Latin names because they were named a long time ago when Latin was more widely used. Here's what those names mean in modern English.

Catena Chain of craters
Dorsum Mare ridge
Dorsa Group of mare ridges
Lacus Lake
Mare Seas
Mons Mountain range
Montes Mountain range
Oceanus Ocean
Palus Marsh
Promontorium Cape
Rima Fissure
Rimae Group of fissures
Rupes Escarpment
Sinus Bay
Terra Landmass
Terrae Highlands
Vallis Valleys

Lunar Atlases

There are many atlases and wall charts vying to help you find the various craters, mountains and features of the Moon. They have their strengths and weaknesses, and you'll find some easier to use than others.

Watch out for any that flip the Moon so that south becomes north, or make any other change to orientation. These are fine for seasoned astronomers who use a specific telescope setup to observe the Moon, but for those of us who switch between correcting lenses, terrestrial telescopes, binoculars and the like, go for a plain and simple map with north at the top. It's also an advantage to have one with high-quality pictures.

Wall charts are also good for getting a general idea of where things are on the Moon. However, they're less use at the eyepiece unless they're safely wrapped up in a dew-proof coating, so it's worth getting a laminated version.

Experiencing Eclipses

Why do eclipses occur so infrequently? It all has to do with the Moon's tilted orbit.

Over the course of a year the Sun moves across the sky on a path known as the ecliptic. It rises in the east and sets in the west, in essence appearing like it travels around Earth.

If the Moon orbited Earth in this same plane, then each month we would get an eclipse of the Sun (when the Moon passes between the Sun and Earth) and an eclipse of the Moon (when the Earth is between the Sun and the Moon). We don't, however, as the Moon's orbit is tilted at an average of 5 from the ecliptic.

Most months this means that from our point of view on Earth, the Moon moves above or below the Sun at new Moon, and above or below Earth's shadow at full Moon. We only get an eclipse when the Moon's orbit intersects the ecliptic and all three bodies are in the correct alignment.

Due to a fantastic coincidence, the Sun is 400 times bigger than the Moon, but around 400 times further away. This means that they appear to be the same size. The Moon just covers the Sun during a total solar eclipse, allowing us to witness its ghostly outer atmosphere, known as the corona.

Top Ten Moon Sights

Our celestial neighbour has enough to keep astronomers busy for a lifetime, but here are 10 highlights for telescopes and binoculars.

Crater Grimaldi
Size: 173km across
Type: Basin
Appearance: Visible even to the naked eye, this dark basin reveals fantastic detail through binoculars and telescopes, such as eroded walls, ridges and low hills.

Rimae Sirsalis
Size: 425km long
Type: Rille system
Appearance: This series of fault lines is visible even in a small telescope, which will reveal Sirsalis's main crack running straight for over 300km through a cratered environment.

Crater Copernicus
Size: 94km across
Type: Impact crater
Appearance: One of the Moon's recognizable features and the result of quite a recent impact, a scope reveals terraced crater walls and central peaks rising from the floor below.

Vallis Alpes
Size: 155km long
Type: Valley and rille
Appearance: A clean gouge through a mountainous region, the 18km-wide fault line can be easily visible in small scope and binoculars as a dark stripe in a lighter landscape.

Crater Plato
Size: 109km across
Type: Lava-filled impact crater
Appearance: In binoculars and small telescopes the beauty of this crater is its jagged rim with 2km high mountains compared to its smooth lava-filled floor.

Montes Alpes
Size: 3.4km maximum height
Type: Mountain range
Appearance: Through binoculars you will just be able to make out this rangle of peaks; with a telescope they start to reveal really good detail, especially if the terminator is close by.

Montes Teneriffe
Size: 2.5km maximum height
Type: Mountain range
Appearance: When caught in the right angle of sunlight this 110km-long mountain range reveals good detail among its peaks using a small scope and around 150x magnification.

Mons Piton
Size: 2.2km in height
Type: Mountain
Appearance: Lying on its own in the flat region of the Mare Imbrium, use a small telescope when the Sun's illumination is low to reveal the shadow cast by this lone peak.

Rupes Recta
Size: 110km long
Type: Rille
Appearance: This popular target for binoculars and small telescopes is another fault line where the lunar surface suddenly drops by 300m. It's best seen when close to the terminator.

Vallis Rheita
Size: 450km long
Type: Valley
Appearance: A long, wide valley that many think is the result of a sustained meteor bombardment. A small telescope will show the crater Rheita next door has a central peak.

Copyright Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Date Taken: 11/28/2016
Author: BBC Sky at Night Magazine
Category: Astronomy

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