- Why Astronomers Should Observe the Moon
- Three Easy Steps to Observing The Moon With Your Telescope.
- The Lunar 100 Challenge
It’s the most well-known object in our night sky, but few of us backyard astronomers understand how to observe the moon with a telescope 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
Or, do you curse the moon for being too bright to observe anything else?
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 nearest celestial 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 194 natural satellites orbiting planets and dwarf planets
Earth’s moon is the fifth largest of them all (see the chart) and the only one to orbit Earth.
Its diameter of 2,159 miles (3,474 km) makes our moon about a quarter the size of Earth, a giant in comparison to its host planet! If you enjoy facts like this, check out our 22 frequently asked questions about the moon for more (it opens in a new window, so you won’t lose this page)
There are disputes about how many craters the moon has, but there is no shortage of them! There are about 1,800 named features, including 1,500 craters, on the moon’s surface.
If you want more enjoyment from looking at the moon with your telescope, you need to know how and when to discover the best craters it has to offer.
Some are easy to find, whilst others are a challenge even in a large telescope, which is great for all abilities and experience levels. There are so many challenges, it will take years of observing the moon before you run out of things to find.
Three Easy Steps to Observing The Moon With Your Telescope.
In the rest of this article, we’ll guide you through getting the best views of the lunar surface with your telescope, following 3 simple steps:
- The Best Place to See Craters
- The Best Time to Look for Craters
- Which Craters to Look for During Each Lunar Phase
1: The Terminator – The Best Place to See Craters
No, it’s not a cyborg from the future! For our purposes, the terminator is the name given to the line which separates night from day on the lunar surface.
It moves from left to right across the surface of the moon twice between each new moon.
The first time the terminator crosses the moon’s surface – from new moon to full moon – day is right of the terminator and night is on the left. After full moon and until the next new moon, the terminator crosses from left to right a second time, but now day is on its right and night is on the left.
Why is this useful information for moon observing?
Well, the best place to see objects on the lunar surface is on or near the terminator.
Contrast the depth and detail you can see in Mare Crisium (circled in the pictures below) when the terminator is nearby (right) to how flat and low contrast it is in full sunlight (left).
This effect happens because the terminator marks the line on the moon’s surface where the sun is just rising (or setting). You’ll know from your experience that shadows are longer at sunrise and sunset.
It’s these long shadows at the lunar terminator which bring high definition and contrast to features on the moon’s surface, making them much easier to locate and study in detail.
We need to know which is the best phase to see a particular crater because, if we look on the wrong night, it will either be in darkness or have its details washed out in full daylight.
This is why part 2 is all about looking through your telescope at the moon at the right time.
2: The Phases of the Moon – The Best Times to Look for Craters
We’ve just learned about the terminator. Its location changes with the phases of the moon, so it helps our lunar observations to have a grip on what the phases are.
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’).
The moon is tidally locked to us, which means we constantly see the same face and not the ‘dark side’ (which has just as much daylight as the side we see). Every month, we see the steady rise and set of the sun over the same features of the moon in a very predictable way.
Our ancestors split the moon’s 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.
Table of Phases of The Moon
|Phase||Rise/Set Time||Days in Cycle||Image|
|New Moon||Sunrise / |
|Pre midday / |
|Midday / |
|Post midday / |
|Full Moon||Sunset / |
|Pre midnight / |
|Midnight / |
|Post midnight / |
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
In the table, we’ve included a brief guide to when you can see each phase. Put simply, the new moon is up all day and the full moon is up all night. In between those, the waxing moon gradually moves from day to night and the waning moon moves from night to day.
What time of night you should point your telescope towards the moon depends on which craters you want to find.
3: Which Craters to Look for During Each Lunar Phase
Now you’ve seen the eight phases of the moon’s cycle, we’ll look at each of them in more detail and see which are the best craters to look for at that time.
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.
In that case, you may wonder, why isn’t there a solar eclipse every new moon?
Well, it’s because the Earth, moon and sun don’t quite sit in the same horizontal plane, they can be as much as 5° apart. The moon and sun are in the same direction, but one appears to be ‘above’ or ‘below’ the other, i.e. they don’t line up.
The new moon rises and sets with the sun, so 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 no features to be seen on the lunar surface at new moon phase.
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. During its waxing crescent phase, the moon rises later in the morning than the sun but still sets before midnight.
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°, or one quarter of its orbit around the Earth, which is where the name of this phase comes from. It is now lit directly 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.
It’s also a great phase to phase to make the most of evening observation time because the first quarter moon rises around midday and sets again at midnight.
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 but don’t worry, a new set of craters are ready to be observed.
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 during this phase, but still before sunset. It will set in the early hours of the morning 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?
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.
It is more frequent that a solar eclipse though. There are at least 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 one 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
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.
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 socially more difficult to observe the moon in this phase as it begins to move out of our night sky and into the morning.
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.
Observing times become more anti-social at last quarter, unless you like an early start! The moon rises around midnight and sets around midday.
Waning Crescent, Day 23-29
Finally, we 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. Eventually, however, the crescent moon becomes too thin for us to see as it moves closer to the sun.
The waning crescent moon rises in the early hours of the morning before the sun and sets in the afternoon before the sun does
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.
The Lunar 100 Challenge
What’s next on your moon observing journey?
Are you the kind of astronomer that likes a challenge?
Many years ago, Sky & Telescope magazine developed a moon observing challenge called The Lunar 100. This sets out 100 objects on the lunar surface for backyard astronomers to observe.
It starts very easily, but steadily gets more and more challenging, with the last 10-20 objects being really quite tough to see.
If that sounds like your kind of thing, why not take a look at the detailed guide we’ve created to help you find all 100 objects.