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The Basics of Lunar Observing
The Basics of Lunar Observing

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!

Explore the seas and craters that texture the lunar surface with our beginners' observing guide.

Sea of Serenity by Craig Heaton

Sea of Serenity by Craig Heaton

The Moon is an ideal object to begin your observing odyssey because it is big, bright and covered with amazing detail. But the thing that surprises most novice observers is the variation it holds. Though the same hemisphere faces Earth at all times, what you can see on the Moon changes from one night to the next.

You may be forgiven for thinking that full Moon is the best time to examine our close companion — not so. While this is a good time to see the long, bright rays of ejecta surrounding prominent craters such as Tycho, the high altitude of the Sun in the lunar sky means no shadows are cast, resulting in a washed-out view of the Moon.

In general, the best time to view a given lunar feature is when the terminator, the demarcating line that separates lunar day and night, is nearby. This is the region where the Sun is either rising or setting, where crater rims and mountain peaks stand out in stark relief, casting inky black shadows across the lunar surface that exaggerate their presence. Those further from the terminator show hardly any shadows and are harder to make out.

At day zero of the lunar cycle — new Moon — the whole of the dark lunar hemisphere points towards Earth. Over the next 15 days the terminator slowly creeps across the lunar surface from east to west until the disc is fully illuminated at full Moon. Then the tables are reversed as the encroaching darkened hemisphere heads west with each passing day, until the diminishing crescent becomes lost in the pre-dawn twilight.

Peering Beyond the Limb
The nature of the Moon's orbit generates another effect that is a boon to lunar observers, a rocking and rolling motion that we call libration. The Moon's orbit is elliptical, and as a result its distance from Earth does not remain constant. When closest it speeds up slightly; when more distant it slows down. This small variation is enough to cause the Moon to 'nod' back and forth on its axis, giving us an occasional chance to see a little more around its eastern and western edges.

The 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. Taken together, this libration allows us to see a total of 59 percent of the Moon's globe, revealing tantalising features normally hidden from view.

With the naked eye it's easy to see the progression of lunar phases, full disc effects such as earthshine and the major lunar seas. Binoculars increase the detail you'll see: as well as dark seas, you'll now be able to spot individual craters and large mountain ranges, especially when they are close to the terminator. The smallest craters you'll be able to pick out will depend on how still you can hold your binoculars, but a pair of 7x50s should comfortably reveal features down to about 50km across.

A telescopic view of the Moon is amazing and one that never gets old. At low magnifications, the amount of detail visible is breath-taking, especially close to the terminator where relief shadows really help to emphasise the detail. Upping magnification by using shorter focal length eyepieces will get you in closer and give you opportunity to 'roam' around the lunar landscape.

Trifles and Troubles
The view you have of the Moon through a telescope will differ from what you see with the naked eye or binoculars depending on its optical arrangement. Through a refractor or compound instrument, the Moon will appear flipped west to east, while through a reflector the image will be inverted.

If you look at the Moon with a telescope you may also notice the surface appears to gently wobble or sometimes even shimmer. This effect is caused by air moving through the atmosphere of our planet, and the greater the turbulence the worse the views.

Such 'seeing' conditions can vary from minute to minute and night to night. The best views will always be had when the seeing is steady and these undulations are less intense; poor seeing, on the other hand, results in loss of detail and fuzzy lunar features.

For centuries, telescopic observers have also reported seeing short-lived changes in brightness on the surface of Moon, events that are collectively referred to as transient lunar phenomena, or TLPs. They have been described as luminous spots that suddenly appear and vanish, localised patches of colour and temporary blurring or misting of the Moon's fine surface detail. However, despite several high-profile reports — including those from Sir William Herschel in 1787 and French astronomer Audoiun Dollfus in 1992 — their existence remains debated to this day.

The problem is that TLPs, being transient by nature, are hard to independently verify and impossible to reproduce. Most are spotted by lone observers, or are only witnessed from a single location on Earth, casting doubt on whether they truly occurred at all. Some believe that TLPs are little more than the result of poor observing conditions or equipment issues. Assuming they do occur, the most popular theory to explain them is residual outgassing from below the lunar crust.

What does seem clear is that TLPs, whether real or imagined, are more prone to occur on some areas of the lunar surface than others, with more than one-third of official reports coming from the region around the Aristarchus plateau.

The Many Guises of the Moon
Even to the naked eye, our satellite is a beguiling subject.

Earthshine
The Moon is not solely lit by sunlight. When it is in a slender crescent phase in the evening or dawn twilight, it's sometimes possible to see its dark portion gently glowing due to sunlight reflected off the oceans and clouds of planet Earth. This effect is known as earthshine. Our planet actually reflects more light onto the lunar surface than the Moon gives us when it is full.

Lunar Halos
On frosty nights, often when the Moon is or near full, you may be able to spot a faint ring of light caused by ice crystals refracting the moonlight in the upper atmosphere. Since the ice crystals are normally all hexagonal, the ring is almost always the same size; it has a diameter of 22. Sometimes it is also possible to detect a second ring, 44 in diameter.

Red Moon
There are two reasons the lunar disc may take on a ruddy hue. The first is if it is low in the sky, so light reflected from it passes through more of our atmosphere. Blue and violet light is scattered more easily, so we see a redder Moon. The other is during a total lunar eclipse: longer sunlight wavelengths are refracted by the Earth's atmosphere onto the eclipsed Moon.

Supermoon
A supermoon is a full Moon that coincides with the closest point to Earth in its orbit, causing the lunar disc to appear larger by as much as 14 per cent. The word is rooted in astrology but, given the correct astronomical term is a 'perigee-syzygy Moon', you can see how it caught on. A supermoon also occurs with a new Moon at perigee — but you aren't able to see this one.

The Big Myth — The Moon illusion
Look for the Moon when it is low to the horizon and you may get the impression that it is unnaturally large — this is the phenomenon known as the Moon illusion, and it appears to be more pronounced around full Moon when the maximum area of its disc is illuminated. In reality, the Moon has more or less the same apparent diameter of around 0.5, whether it is looming over the horizon or riding high in the sky.

One explanation for the illusion arises from our perception of the shape of the celestial sphere above us; instead of a hemisphere, we perceive the sky to be a flattened dome. Consequently the lower the Moon is in the sky, the farther away and larger it is perceived to be. When the Moon is high in the sky we conversely perceive it to be closer to us and therefore smaller in apparent size.

Few people seem to be immune to the Moon illusion, even though the viewer may be fully aware that for any given evening there is actually no appreciable difference in the Moon's apparent diameter, regardless of its height above the horizon.

The Rarest Moon
No doubt you've heard the expression 'once in a blue Moon' — meaning something that is exceptionally rare. But what exactly is a blue Moon, and does our neighbour ever adopt an azure appearance?

When astronomers use the term, they are most likely referring to one of two lunar events — neither of which cause the Moon to turn blue.

Traditionally, a blue Moon is considered to be the third full Moon in a season that has four. Normally, there are only three. The second and more modern interpretation is that it is the second full Moon that occurs in a calendar month, which can happen as a lunar cycle only takes 29.5 days to complete.

Why the discrepancy in definitions? It appears to be the result of a publication mistake that appeared in 1946 that confused the traditional meaning, which dates back to 19th-century editions of the Maine Farmers' Almanac.

And yet there is circumstance that can cause the Moon to truly appear bluish, as it did in the wake of the Krakatoa eruption in 1883, and it is exceptionally rare. The secret is that the atmosphere needs to be flooded with dust particles of a specific size — slightly smaller than the wavelength of red light — and that size alone. These particles scatter red light, causing the Moon to take on a slight cerulean cast.

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.

Details
Date Taken: 07/29/2016
Author: BBC Sky at Night magazine
Category: Astronomy

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