Patrick Moore’s guide to observing the Moon

Interested in observing the Moon but not sure where to start? Take advice from the late, great Sir Patrick Moore.

The Moon, as seen by the Lick Observatory. Credit: Lick Observatory

The best time to observe the Moon depends upon the particular interests of the observer. In general, however, full Moon is the worst time for observing. This is because there are almost no shadows and all features are drowned by the rays.

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During this phase, a crater is hard to identify unless it has a dark floor (as is the case for Plato – one of the Moon’s most conspicuous craters) or if it is very bright (as with Aristarchus).

Conditions for observing tend to be best during the crescent, half or gibbous phases. For the Earthshine, of course, a crescent stage is absolutely essential.

Phases of the Moon.

Crescent Moon (left)

  • Terminator just past the Theophilus chain
  • Day 6 after new
  • The whole of the Mare Serenitatis is on view, as well as the Mare Tranquillitatis. Look for the great chain of Theophilus, Cyrillus and Catharina.

Gibbous Moon (middle)

  • Terminator past the Ptolemaeus chain
  • Day 8 after new
  • The Apennines are coming into view, with part of the Mare Imbrium. Look for Plato, the trio of Archimedes, Aristillus and Autolycus, and the Alps.

Waning Moon (right)

  • Terminator near Plato
  • Day 21 after new
  • The western side of the Moon is prominent, as is Plato and the ray-crater Copernicus. Look out for Aristarchus, the Moon’s most brilliant crater, and Grimaldi, which has the darkest floor.
An image of Aristarchus Plateau, as seen by the Lunar Reconnaissance Orbiter. The patch lies northwest of the Aristarchus crater, seen in the bottom right. Credit: NASA (image by Lunar Reconnaissance Orbiter)
An image of Aristarchus Plateau, as seen by the Lunar Reconnaissance Orbiter. The patch lies northwest of the Aristarchus crater, seen at bottom right. Credit: NASA (image by Lunar Reconnaissance Orbiter)
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A closer look

Binoculars can give splendid views of the Moon and will show many of the features given in the outline map.

Of course, low-power binoculars (say x7) will show less detail, but are excellent for the Earthshine and lunar eclipses, though probably not for occultations of any but the brightest stars.

Higher magnifications give greater scope, but above about x12 the binoculars are so heavy that a neck mounting or a tripod has to be used.

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It’s not the size of the scope…

One can now obtain small telescopes at low cost that will give satisfying views of the Moon – something that could not have been said a few years ago.

For specialised investigations, I would say that an aperture of 4 inches (10cm) for a refractor or 6 inches (15cm) for a Newtonian reflector is about the minimum requirement, and this does depend upon the quality of the optics.

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The Moon’s definition

Filters are widely used and for some types of observations they are invaluable.

But as far as the Moon is concerned I have decidedly mixed feelings about them, because I always feel that they result in a slight loss of definition.

However, if you want to see a full or near-full Moon under a reasonably low power a filter will generally be an advantage, as it will cut down the light and prevent hopeless dazzle.

Otherwise, I prefer to use higher magnification.

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The limits of magnification

Never use too high a magnification.

Generally, a power of about x50 per inch of aperture is the maximum, but if the image is even slightly blurred it is wise to change at once to a lower magnification.

The highest power eyepiece you have for your telescope will probably have to be reserved for nights when conditions are really excellent – and in Britain these nights do not happen too often.

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Opportunities mist

A night that is slightly misty may be good for the lunar observer, though of course it is of no use for deep-sky objects such as nebulae.

Slight mist will often produce steadiness, and this is probably the main requirement for anyone observing the Moon.

However, any appreciable thickness of cloud will be fatal.

Do not try to carry on when conditions are poor, as errors will undoubtedly creep in.

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Stop stopping

Stopping down is something I never do.

I know that Percival Lowell stopped down his 24-inch refractor to a mere 18 inches, and I have heard claims that in our climate no telescope of aperture over about 12 inches is suitable, but I disagree.

If you find that conditions are not good enough for you to use your telescope at full aperture, then give up until conditions improve.

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On a budget

Good photographs can be taken with equipment that is not too expensive: webcams are affordable for most.

Electronics have largely taken over from photography and amateurs can match the best results obtainable from professional observatories only a few decades ago.

If you want to use this sort of equipment, it is wise to save up and buy something worthwhile. After all, it may well last you a lifetime.

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Altitude

In general, the greater the altitude of the target object, the better the conditions will be, with less tremor and blurring.

This applies to the Moon, but during the slender crescent phase there is no alternative, and all one can do is wait for the best compromise between altitude and sky brightness.

Even when the sky is light, soon after sunset or before sunrise, conditions can often be tolerable.

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Don’t wait

If you have been making a drawing of a lunar feature, do not wait for too long between checking at the telescope, as the shadows can cause marked changes in aspect over a short period.

Add the following data to an observation: date, time (GMT, never Summer Time), equipment used, and seeing conditions, usually on the Antoniadi scale.

10

Never send away an original observation.

Sooner or later something you really need will go astray in the post.

Keep your observation log and send away copies. Collaborate with other observers and join a society: the British Astronomical Association has a history and reputation second to none, but many local associations now have flourishing lunar sections.

The isolated observer will not accomplish much.

Video: a NASA visualisation of the Moon’s phase and libration at hourly intervals throughout 2019, as viewed from the Northern Hemisphere. Credit: NASA Goddard

Explaining the Moon’s wobble

The Moon completes one orbit in exactly the same time that it takes to make one rotation on its axis.

The result is that the Moon keeps the same face turned toward us all the time and we never see its far side.

But though the rate of spin is constant, the orbital velocity is not: the Moon moves fastest when it is closest to the Earth.

Therefore the amount of spin and the position in orbit become regularly ‘out of step’ and the Moon seems to rock slowly and slightly to and fro.

As a consequence, we can see a little way round one mean limb, then the other.

This is known as libration in longitude and allows us to examine 59 per cent of the surface, though no more than 50 per cent at one time.

A lunar jargon buster

Mare Latin for ‘sea’, though the lunar maria were never water-filled – they are old lava plains. The Latin names, such as Mare Nectaris (Sea of Nectar), are used.

Terminator Boundary between the sunlit and dark hemispheres of the Moon.

Rill (or Rille, or Cleft) A crack-like feature, sometimes associated with a craterlet-chain.

Albedo Reflecting power. On the Moon, the highest albedo feature is Aristarchus; the dark floor of the crater Grimaldi has the lowest albedo.

Antoniadi scale A scale for seeing conditions, devised by the Greek astronomer EM Antoniadi. It runs from ‘one’ (perfect, which is hardly ever encountered) down to ‘five’ (so poor that observing will usually not be attempted).

Perigee This is the point in the Moon’s orbit when it is closest to the Earth. This is 221,460 miles (356,405km).

Apogee This is the point in the Moon’s orbit when it is furthest from the Earth. This is 252,700 miles (406,681km).

Sidereal Period The time taken for the Moon to complete one orbit (27.321661 days).

Synodic Period (lunation) This is the interval between successive new Moons, or successive full Moons (29 days, 12 hours, 44 minutes and 3 seconds).

Ray-craters Craters which are the centres of systems of bright rays or streaks. The rays are not seen under low solar illumination and are surface deposits, overlying all other formations.

A view of Mare Humorum captured by the Lunar Reconnaissance Orbiter. On the eastern edge can be seen the cracks known as Rimae Hippalus. Credit: NASA (image by Lunar Reconnaissance Orbiter)
A view of Mare Humorum captured by the Lunar Reconnaissance Orbiter. On the eastern edge can be seen the cracks or rilles known as Rimae Hippalus. Credit: NASA (image by Lunar Reconnaissance Orbiter)

Transient Lunar Phenomena explained

Dormant though it is, mild activity is not unknown on the lunar surface in the form of localised glows and obscurations.

These events are known as Transient Lunar Phenomena (TLP) – a term for which I believe I was originally responsible.

I first saw a TLP as long ago as 1939, when I recorded an obscuration in the southern part of the Mare Crisium.

Until recently there was considerable official scepticism about them, mainly because so many of the reports came from amateur observers, but their reality is no longer in doubt.

TLPs seem to occur mainly in areas rich in rills and near the borders of the Circular Maria – the brilliant crater Aristarchus is particularly subject to them.

There have been some professional reports. For example on 3 November 1958, while using the 50-inch reflector at the Crimean Astrophysical Observatory, the Russian astronomer NA Kozyrev saw a red glow inside the crater Alphonsus and even obtained a spectrum.

On 30 April 1966 glows and streaks inside the crater Gassendi were seen by several observers, including myself (with a 12.5-inch reflector).

TLPs are not common, but several hundred were included in a catalogue published by NASA in 1968.

Final confirmation came in 1992 in an observation by the French planetary observer A Dollfus (Paris Observatory).

He wrote: “On 30 December 1992 glows have been recorded on the floor of crater Langrenus. They are apparently due to dust grain levitations above the lunar surface, under the effect of gas escaping from the soil. The Moon appears as a celestial body which is not totally dead.”

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This article originally appeared in the June 2005 issue of BBC Sky at Night Magazine. The late Sir Patrick Moore was the original presenter of The Sky at Night and one of Britain’s most beloved astronomers.