Observing the Moon
Follow the tips below to maximize your lunar viewing experience.
1. Understanding the Phases of the Moon
The moon is the brightest object one can see with the naked eye in the sky. The moon is actually visible during the day as well! My favorite way to keep track of the Moon’s phases is with this wall poster.
The moon is a ~3,500 km ball of rock orbiting the Earth at an average distance of around 385,000 km away. Due to its size and distance from Earth, it appears half a degree across in the sky. The moon takes 27 days or about a month (the word month comes from the moon) to complete one orbit. The moon’s phases are a result of its orbit around the Earth and us seeing a different fraction of the sunlit side depending on its location in orbit. This video by NASA Goddard shows all the moon phases for 2023.
During a new moon, the moon rises and sets the same as the sun. Two to three days after a new moon a thin crescent will be visible shortly after sunset. As the month goes on from the new moon you can watch the moon rise about an hour later every day as it grows to a half moon within a week. A growing moon is referred to as waxing. The moon will continue to grow in size for another week until the full moon, rising right after sunset. The moon will then begin to shrink over the next two weeks, rising later in the night until it once again rises and sets with the sun during the next new moon. A shrinking moon is referred to as waning.
The Phases of the Moon
2. Best Times to Observe
Observational astronomy is all about improving contrast, and if imaging, more precisely increasing the signal-to-noise ratio. Views with higher contrast between features on an object or with an object and the background sky allow you to see the most detail.
The area of the moon with the highest contrast is the terminator, the line dividing the day and night sides. The areas of the terminator are experiencing sunrise or sunset providing the best view of details on the surface with the hills, mountains, and craters casting long shadows allowing them to easily be picked out. Notice how in the image of the moon (right on desktop, above on mobile) the craters and surface details increase in visibility as you get closer to the terminator. This makes the crescent, half, and gibbous moon to be better times to view the moon, with the half moon being the best. The full moon is evenly illuminated washing out the details on the surface as higher and lower areas look the same.
The Half Full Moon
3. Crescent Moons and Earthshine
Viewing a crescent moon is best around the new moon and shortly after sunset or before sunrise. The moon appears as a crescent the week before and after a new moon. Two days before and after a new moon, it is usually too close to the sun to see against a bright post and pre-sunset sky. The crescent moon has great significance in many cultures and the sighting of a crescent moon dictates the start of the month for Islamic calendars. Viewing a crescent moon does not usually require a filter to reduce the brightness of the view.
When the moon is a crescent only a sliver is illuminated by the sun. However, if you look closely you can actually see the night side of the moon an hour after sunset allowing you to see the entire Earth-facing side. This is called Earth Shine due to the night side being illuminated by light reflecting off the Earth. Just like how a full moon lights up the night on Earth, a crescent moon has a near “full Earth” lighting up the sky. The Earth is 3.6 times larger than the moon and 4.2 times more reflective beaming the night side of the moon with the light of 55 full moons. Earth Shine can be into the gibbous phases as well but becomes progressively harder to do so as the full moon approaches.
The Crescent Moon with Earth Shine
4. Half Moon
The half-moon is the best time of the month to observe surface details, mountains, and craters. At high magnifications, individual mountains, mountain ranges, and crater central peaks can be seen along the terminator casting long shadows. Over the course of 10 minutes, you can see the shadows grow or shrink as the sun rises or sets for the part of the moon.
When viewing the moon at low magnifications (equivalently also large exit pupils) in a telescope the moon will begin to appear uncomfortably bright around this phase. A neutral-density filter that screws onto the end of an eyepiece is the best way to reduce the amount of light in the eyepiece. I prefer to use polarizing filters over fixed transmission filters as they are only marginally more expensive but much more versatile. Your telescope may have a removable cap in the cover allowing for a smaller opening to reduce the amount of light entering. In my experience, this method will dim the image but reduces contrast and resolution.
Telescope eyepiece view of the half full Moon at 75x
5. Gibbous Moon
The gibbous moon is the most overlooked phase, seen as not as interesting as a crescent with Earth Shine, or a half moon with high contrast and countless craters along the terminator. A polarizing filter is a must unless high magnifications are used.
Earth Shine is still visible during gibbous phases near the half-moon and can be a fun challenge to try and see. Due to slight asymmetry in the shape of the moon, the shape of gibbous moons before the full moon is actually noticeably different with the difference visible but small.
From the half-moon phase onwards the sky begins to be washed out from moonlight. A gibbous moon prevents viewing of faint deep sky objects when in the sky and very noticeably reduces the visibility of dimmer stars.
The Gibbous Moon
6. The Full Moon
A rising or setting full moon is always a stunning sight to see. On the night of a full moon the sky never truly gets dark appearing with a twilight glow all night. The full moon is twice as bright as a 95% illuminated moon due to the way light scatters off of the lunar surface. All but the brightest deep sky objects become hard or impossible to visually see. Use a polarizing filter when viewing the full moon through a telescope. The full moon is bright enough to make your eye feel warm in larger telescopes when unfiltered at low magnifications. This is still safe but is usually very uncomfortable.
Not all full moons are equal. The moon’s orbit is elliptical with the moon coming closer and moving away from us throughout its orbit. When the moon is closest near full it is a “Super Moon” appearing up to 15% larger than when farthest away. A full moon the hours before and after a total lunar eclipse will also appear to be brighter than a typical full moon as it is more directly opposite the sun than an average full moon.
The Full Moon
7. Viewing the Moon with a Telescope
Telescope eyepiece view of craters on the moon at 400x
The moon is one of the few objects that always looks better live through a telescope than an image. At low magnifications, a polarizing filter can help reduce the image brightness to a comfortable level. If you are having trouble pointing at the moon, as long as you are close you can follow the glow in the sky toward the direction of increasing sky brightness.
The moon looks great at any magnification from low to high. Low magnification is great for viewing Earth Shine, lunar eclipses, and the disk as a whole. A binoviewer allows you to use both eyes with a telescope and the moon looks absolutely stunning in one. Anyone who has seen the moon in a binoviewer will vividly remember the view, myself included. Watch this video by Ed Ting for more information on binoviewers.
At high magnifications enjoy looking at surface details such as craters, mountain ranges, and more!
8. Conjunctions and Occulatations
The Moon in Conjunction with Saturn an hour before an occultation
A conjunction is when two objects in the sky appear very close to each other in the sky. When one object eclipses another blocking the view of the eclipsed object it’s called an occultation. Since the moon travels around the entire sky in a month it is in conjunction with and occulting many objects every night.
However, the moon is bright so typically the visually interesting conjunctions and occultations tend to be with bright objects such as the planets or brighter stars.
Conjunctions of the moon with planets are usually easily visible to the naked eye. Due to how bright the moon is, occultations tend to be very difficult to see with the naked eye alone due to the dimmer object being obscured by the glow of the moon. Using a pair of binoculars or a telescope fixes this issue.
If you spend enough time looking at the moon you are bound to see a plane fly in front of it, and the ISS (International Space Station) does so as well! You can use this transit finder to find when the ISS flies over the moon or sun near your location.