Introduction to Observing the Moon
It's the most well-known object in our night sky, but few astronomers understand how to observe the moon for the most enjoyment.
Does this moon observing pattern sound familiar to you:
- On a random night, point telescope towards the moon
- Look at some craters, admire their beauty and wish you knew their names
- Get frustrated or bored of not knowing what you're looking at
- Move on to look at a planet or galaxy
- Curse the moon for being too bright for us to see a planet or galaxy?
If so, you are missing out on the simplest pleasure in amateur astronomy...
The detailed observation of the surface of another world!
In this article, we're going to show you just how easy it is to get an incredibly rewarding relationship with our neighbor.
- How to plan an evening's moon viewing
- The best features to look for... and when to see them
- How to get deeper into your journey of lunar discovery, and
- how to keep track of your progress.
Why Astronomers Should Observe the Moon
To get the most enjoyment from observing the moon, we need to understand just a little about how it works.
There are 181 natural satellites orbiting planets and dwarf planets in our solar system, 19 of which are large enough to look round like our moon.
Earth's Moon is the fifth largest satellite in the solar system.
Its diameter of 3,474 km (2,159 miles) makes our moon about a quarter the size of Earth, a giant in comparison to its host planet!
There are disputes about how many craters the moon has, but...
If you want to observe the moon in a new light, you need to know just how and when to discover the best craters it has to offer.
And there is no shortage of them!
There are about 1,800 named features, including 1,500 craters, on the moon's surface.
Some are easy to find, whilst others are a challenge even in a large telescope.
But this is great news!
Because it's easy to get started finding your way around the moon! But, there are so many challenges it will take years of observing the moon before you run out of things to find.
Why the Moon's Phases make for AMAZING Observations
The time from one New Moon to the next is the moon's synodic period. At 29.5 days, it's very close to a calendar month (which is why the word 'month' comes from 'moon').
As the moon circles the Earth, we constantly see the same face: it is tidally locked to us.
Every month, we see the steady rise and set of the sun over the same features of the moon in a very predictable way every 29.5 days.
Our ancestors split the 29.5-day cycle into phases which match how much of the moon's surface is lit by the sun. The chart below names each of the seven phases and shows when they occur.
Image Credits Below
You can see from the pictures that the waxing moon lights up gradually from its left side towards its right.
After reaching full, the moon wanes and darkness creeps from left to right across the lunar surface. Just before the new moon, all that's left is a thin crescent on the moon's left-hand limb.
Why the Terminator is Vital for Moon Observing.
No, it's not a cyborg from the future! Instead, it is the name given to the line which separates night from day on the lunar surface.
It moves from right to left between new and full moons and then left to right as the moon wanes from full to new again.
The first time the terminator crosses the moon's surface, it is day on the right and night on the left. After full moon, the terminator crosses from left to right a second time, but now day is on its left and night is on the right.
Why is this useful information for moon observing?
Well, the best time to see an object on the lunar surface is when it is on or near the terminator.
Just take a look at the depth and detail you can see in Mare Crisium (circled, right) when the terminator is nearby. Compare that to how flat and low contrast it is when in full sunlight (circled, left). Click the picture to see a bigger version.
This effect happens because the terminator marks the line on the moon's surface where the sun is just rising (or setting). As you know from experience on Earth, shadows are longer at sunrise and sunset.
Long shadows bring a real definition and contrast to features on the moon, making them much easier to locate and study in detail.
Now you see why we need to know which is the best phase to see a particular feature, such as a crater. If we look on a wrong night, it will either be in the dark or washed out in full daylight.
If you plan properly and look on the one or two nights when the terminator is nearby, you will see the feature in its very best light.
And, if you're not sure where to begin, you can always try and complete the classic moon challenge: the Lunar 100.
When are the Phases of the Moon?
Now you've seen the seven phases of the moon's cycle, we'll examine each of them in more detail.
As you read the detail on each phase, note the time of day it is visible. Nearly-new moons are mostly visible in our daytime, whereas fuller moons spend more time out at night.
New Moon, Day 0
The moon sits directly between the Earth and the sun, so no sunlight hits the surface of the moon which faces us.
Why isn't there a solar eclipse every new moon then?
It's because the Earth, moon and sun don't quite sit in the same plane. They can be as much as 5° apart.
The new moon rises and sets with our sun in the sky, i.e. it is 'up' all day and hidden all night. But, we never see it because its unlit surface is lost in the brightness of daylight.
There are rare moments when the Earth, moon and sun do sit on the same plane at new moon. This is when we have a solar eclipse, which is the only time we can see the unlit surface of the new moon.
New Moon - Fact Box
Phase Day: 0 / 29
Surface Illumination: 0%
Moon Rise: With the sun
Moon Set: With the sun
What to look for: Try to find the youngest new moon that you can. Look for the thin crescent with 36 hours of new as it sets in the west just after the sun.
The best time of year to try this is late March, when the ecliptic's steep angle with the horizon pushes the moon higher in the sky.
Waxing Crescent, Days 1-6
The sun is beginning to rise on the surface of the moon.
The sliver of crescent steadily fattens out (waxes) over the first week of the lunar cycle as the sun moves out from behind the moon.
In fact, the moon's orbit is moving it from directly in front of the sun to 90° around, so the sun bathes its right-hand side in daylight.
For observing, the moon is gradually moving from a daytime to a nighttime object. It now rises later in the morning but still sets before midnight.
Waxing Crescent - Fact Box
Phase Day: 1-6
Surface Illumination: 1-49%
Moon Rise: In the morning, after sunrise
Moon Set: In the evening, before midnight
What to look for: In the earlier part of this phase, try and see 'earthshine' which is when the dark part of the moon reflects light from the Earth.
Later on, try and see some of the bigger craters, like Piccard and Vendelinus, highlighted on the map, above.
First Quarter, Days 7
As the name suggests, a quarter of the moon's cycle has now passed and we can see the right-hand side of the moon fully illuminated.
Now the moon has travelled exactly 90° of its orbit around the Earth and is directly lit from the right by the sun.
The terminator splits the moon neatly in half at first quarter and there are some great sights to observe (click on the image for detail).
First (and last) quarter is the most dramatic time to look at the moon in detail. The shadows cast by the sun at the terminator are at their longest and pull the scenery into stunning relief.
In terms of observation time, the moon is now rising around midday and will set again at midnight.
First Quarter - Fact Box
Phase Day: 7
Surface Illumination: 50%
Moon Rise: Around midday
Moon Set: Around midnight
What to look for: The major Mares (seas) are now in view, Serenitatis and Tranquillitatis.
There is a wealth of lovely caters running all down the terminator at first quarter, click on the image above to get around 20 to take a look at!
Waxing Gibbous, Days 8-14
Daylight continues its march across the surface of the moon.
In the waxing gibbous phase, the moon progresses from half illuminated to almost fully lit (full moon).
A lot of the features you've been able to study in the last few days are now washed out in the sun's bright glare, and a new set of craters are ready to be observed (click the image to enlarge it).
In its orbit around Earth, the moon is moving 'behind' us in relation to the sun, which also means it's moving into our nighttime sky.
Expect the moon to rise later in the afternoon now, but still before sunset. It will set in the early hours of the morning, just before sunrise.
Thanks to its evening appearance, waxing gibbous is one of the best phases for amateur astronomers to study the moon.
To get the most reward from your lunar observations, pick a small area to get stuck into. Learn the details of it and see what's the smallest feature you can distinguish with your telescope? Study a mountain range in detail - can you make out any individual peaks?
Waxing Gibbous - Fact Box
Phase Day: 8-14
Surface Illumination: 51% - 99%
Moon Rise: In the afternoon, before sunset
Moon Set: In the morning, before sunrise
What to look for: The infamous Tycho is a great spot during this phase.
The craters Helicon and Le Verrier, to the right of Sinus Iridum makes for a sight you can be proud to locate with your telescope!
Full Moon, Day 15
The moon is now on the opposite side of Earth to where it started as new.
Sitting 'behind' Earth now, the sun beams straight onto the face of the moon, illuminating it 100%.
If the Earth, moon and sun are in the same plane when this happens, we see a lunar eclipse. Earth passes directly between the sun and moon, casting its shadow over the moon's surface.
Most lunar cycles this does not happen as the three bodies are not in the same plane.
It is more frequent that a solar eclipse though. There are around two lunar eclipses every year, and there can be up to five.
Since the moon is directly opposite the sun when it's full, we can't see them both in the sky at the same time. The full moon rises at sunset and it sets as the sun rises. You'll never see a full moon in the daytime!
This phase is the worst for astronomers for two reasons
- The surface features are all washed out because there are no shadows, and
- The incredibly bright moon (11x brighter than first or last quarter) is out all night, so other objects are harder to see
There is one feature which does look its best during a full moon... lunar rays.
Bright Lunar rays surround some craters on the moon. They are caused by material blasted out of the ground as a meteor hit the surface and formed the crater.
The crashing meteor disintegrates in the process, leaving a crater. The material dislodged in the strike settles in streaks around the crater itself, which we can see as rays.
Tycho and Copernicus are the most famous examples of craters with rays, click the image above for a bigger view of where to find them on the moon's surface.
Full Moon - Fact Box
Phase Day: 15
Surface Illumination: 100%
Moon Rise: At sunset
Moon Set: At sunrise
What to look for: The bright lunar rays of Tycho, Copernicus and Kepler craters
Waning Gibbous, Day 16-21
We're past the peak brightness now and night is beginning to fall on the lunar surface.
In its orbit around Earth, the moon has passed 'behind' us and is working its way back to being in front. Illumination from the sun now comes from the left-hand side.
The terminator moves from right to left each day for the second time since new moon, gradually pulling night all across the surface of the moon.
It becomes more difficult to observe the moon in this phase as it begins to move out of our night sky and into the morning.
Objects to look for are the same as those for the waxing crescent phase because the terminator is in the same place. However, day and night are now opposite to the picture, i.e. day is on the left and night is on the right.
Waning Gibbous - Fact Box
Phase Day: 16-21
Surface Illumination: 99%-51%
Moon Rise: After sunset, before midnight
Moon Set: After sunrise, before midday
What to look for: See Petavius and Langrenus' central crater peaks illuminated from a different angle.
Last Quarter, Day 22
Swinging its way back to where we began, the moon has now reached last (third) quarter.
At this point, the sun shines directly on its left side and the terminator once more splits the lunar surface into two perfect halves of day and night. This time with day on the left and night on the right.
The features to see are the same as those at first quarter, but they look quite different when illuminated from the opposite side. You'll also observe details that you just can't see at first quarter.
Try to pick out Mare Tranquillitatis, where the Apollo 11 moon mission landed, setting Neil Armstrong on the moon in 1969.
Observing times become more anti-social at last quarter, unless you like an early start! The moon doesn't rise until around midnight now, so it's the early hours of the morning before it's high enough in the sky to observe well.
It sets about midday.
Last Quarter - Fact Box
Phase Day: 22
Surface Illumination: 50%
Moon Rise: Midnight
Moon Set: Midday
What to look for: This is an excellent time to go crater hunting! Can you find Walter, Werner and Aliacensis lit from the opposite side to first quarter?
Waning Crescent, Day 23-29
We finally work our way back to the new moon as the last of the moon's 'day' slips off the left-hand side.
Entering the final week of the lunar cycle, the moon shrinks to an ever-thinner crescent as it approaches new.
Night continues to cover the lunar surface for a few days, and there are many craters to be seen (click picture for details).
Eventually, however, the crescent becomes too thin, and the moon moves too close to the sun, for us to see it.
The waning crescent moon rises in the early hours of the morning before the sun and sets in the afternoon before the sun does.
The terminator is in the same place now as it is at waxing gibbous.
What's the thinnest crescent you can see? Try looking east an hour before sunrise on the day before a new moon to see a sliver of moon rising above the horizon.
Waning Crescent - Fact Box
Phase Day: 23-29
Surface Illumination: 49%-01%
Moon Rise: After midnight, before sunrise
Moon Set: After midday, before sunset
What to look for: Can you find Mee and Hainz before the crescent gets too thin for you to see them?