What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky

Our Moon

Quick Facts about Our Moon 

• Our Moon is the only natural moon of our planet Earth. It is also the 4th largest moon in our solar system. 

Diameter: 2,160 miles 
Mass: 1.23% of Earth’s mass 
Density: 3.34 where water = 1 
Gravity: 0.165 times that of Earth 
Albedo (% of Sunlight reflected): 11% 
Inclination of Axis to Orbit: 6.7° 

Distance from Earth: 238,856 miles 
Revolution about Earth: 27.322 days 
Inclination of Orbit to Earth’s Orbit: 5.1° 

Atmosphere: No atmosphere 
Surface Temperature: 266° F in Sun to –300° F at night There are many more facts about the Moon at the end of this page. 

Most interesting features in a small telescope: Everyone is enthralled seeing craters at higher magnification around the terminator (the “line” that separates the light side from the dark side—visible in the picture to the right). On “good” nights when the air is steady and using magnifications of 100x to 250x, it really does seems like you are flying over these craters and the detail in and around the rims is amazing!

This image of the Moon shows some color because it was enhanced to bring out the slight differences in color that we cannot see with our eyes.

Detailed Information about the Moon

Diameter: 2,160 miles; 3,476 km which is 27.3% of Earth’s diameter 
Surface Area: 7.4% of Earth’s surface area 
Volume: 2.03% of Earth’s volume 
Mass: 1.62 x 1023 pounds; 7.35 x 1022 kg (1.23% of Earth’s mass) 
Average Density: 3.34 gm/cm3 (water is 1.00 gm/cm3) 

Gravity: 0.165 times the gravity of Earth 
Escape Velocity: 1.5 miles/sec (5,369 miles/hour); 2.4 km/sec 

Average Distance from Earth (measured from the centers of both bodies): 238,856 miles; 384,401 km; 1.3 light-seconds 
Closest  Distance to Earth: 221,457 miles; 356,400 km 
Distance from Earth: 252,711 miles; 406,700 km 
Eccentricty of Orbit: 0.055 or 0.16% out of round. 

Sidereal Revolution Period (One Complete Orbit): 27.322 days 
Synodic Revolution (New Moon to New Moon Period): 29 days, 12 hours, 44 minutes, 3 seconds 
Average Orbital Velocity: 2,287 miles/hr; 3,681 km/hr 

Arc Degrees of Movement in Sky: 0.51° per hour; 12.2° per day 

Inclination of Orbit to Earth’s Orbit: 5.1° 

Rotation Period of Nodes: 18.61 years. The nodes represent the “line” created by the “intersection” of the Moon’s orbit to Earth’s. Its rotation plays a major role in the frequency of eclipses. 
Rotation Period on Axis: The rotational period is the same as the Synodic Revolution, thus the same side of the Moon always faces Earth. 
Inclination of Axis: 6.7° to its orbital plane

Albedo: reflects 11% of sunlight 
Magnitude at Full Moon: –12.6

Temperature Range: –300° F (–184° C) to 266° F (130° C); Poles remain at a constant –140° F (–96° C); One meter under the surface remains at a constant –31° F (–35° C) 

Surface Elevation Range: 11 miles; 17.7 km (lowest to highest points). Both points are located on the far side. The coordinates for the lowest and highest points are not known precisely but one set is as follows: lowest [70° S, 171° E], highest [3.4° N, 160° E]. 

Surface Soil (Regolith) Composition: Oxygen 42%, Silicon 21%, Iron 13%, Calcium 8%, Aluminum 7%, Magnesium 6% and Other 3%. The Moon’s surface has slightly more Iron, Calcium and Magnesium than on Earth’s surface. 

Age: a little less than 4.6 billion years

Farside or Backside of the Moon is NOT the nightside. The farside cycles through day and night and phases just like the nearside facing Earth.

Capitalization: The word Moon SHOULD BE CAPITALIZE because that is its name! The same goes for the Sun.

Our Moon

“Greek myth unknowingly hit upon truth with its story that the goddess of the Moon, Selene, was a descendan​t of Mother Earth or Gaia.”

Of all the celestial objects, the Sun and the Moon command the most attention. The Sun rules by day, and the Moon by night, waxing and waning through phases that make it visually unique. The Moon has been an integral part of humanity since the earliest times, gracing our skies with a splendor unmatched by any other celestial body. I have often wondered what people in the past thought of the Moon, ever changing, disappearing and even obliterating. To me, it would have been absolutely haunting and mysterious. So, I am glad I live in a time when mostly rational thought rules and we can appreciate the Moon for what it is, a planetary satellite, inextricably bound to us by gravity, but beautiful to behold. 

Where did the Moon 
come from? 
The most accepted theory is that the Moon is the result of a Mars-sized object colliding with Earth “shortly” after the formation of our Solar System 4.6 billion years ago. This catastrophic impact caused the Mars-sized object, along with part of Earth’s crust and mantle, to break up into particles that formed a ring orbiting Earth and later coalescing to form the Moon. Crustal and mantle materials from the Earth have been identified in the Moon rocks retrieved from the Apollo landings, lending support to this theory. However, this theory does not explain everything that we know about the Moon and will most likely be modified with greater understanding of the Moon's composition.

The Moon can be seen during the day! 
An astronomy professor I had in college said he regularly received telephone calls from concerned members of the community. They would ask if something was wrong because the Moon was out during the daytime. He always assured them this was normal and that they had probably never noticed the Moon during the day.

When my daughter was two, we were driving around and I pointed the Moon out to her, plainly visible as a white crescent against the blue daytime sky. Regularly after that, and to my surprise, she would stretch out her arm and point her finger, and say only as a two-year old can, “Moooon.” Her ability to find the Moon during the day became uncanny. Once, I told her, “No, that’s not the Moon,” but on closer examination, I noticed that she was right. When you start noticing the Moon during the day, you will see it there often. In fact, about the only time that you cannot see the Moon during the day is around New Moon. 

From New Moon to Full Moon, the Moon trails the Sun. From Full Moon to New Moon, it precedes the Sun. The same side always faces us. 

What’s on the back? 
Why does the same side of the Moon or “near side” as it is often referred to, always face the Earth? Because the Moon’s rotation on its axis (Yes, the Moon rotates on its axis just like Earth!) is synchronized with its revolution around the Earth. This is not a  

coincidence. The gravitational effects of tidal and other forces have “locked in” the heaviest side of the Moon to always face the Earth.

What’s on the other side? The back or “farside,” as it is often called, is covered heavily with craters. There are a few small maria (dark plains), but they are indistinct. The Earth-facing or near side is by far more interesting.

Full Moon Nicknames 
There is probably more lore associated with the Moon than any other celestial body. Every culture has its myths and traditions; however, the Moon’s most important impact is the division of the year into months. In the past, the Moon was given 12 nicknames, one for each month of the year. The nicknames that are occasionally used today include Harvest, Hunter, Moon Before Yule and Moon After Yule (Yule refers to Christmas). The Harvest Moon is the Full Moon closest to the Autumnal Equinox (about September 23). The Hunter’s Moon follows Harvest; next comes Moon Before Yule, then Moon After Yule. 

Once in a Blue Moon 
The origin of the phrase “Once in a Blue Moon” is uncertain, however it means very seldom. The modern day definition of a Blue Moon refers to the occurrence of two full Moons in a month. This happens about once every three years. In the past, when the Moon’s 12 nicknames were used, “Blue Moon” was the name given to the third Full Moon in a season (any of the four seasons) that had four Full

Moons. A season, which spans three months, normally has three Full Moons. 

Man in the Moon 
What about the Man in the Moon? The darker plains (maria) and the lighter cratered areas (terrae) have given rise to people seeing a host of figures in the Full Moon. These include a rabbit, donkey, jack-o’-lantern, woman, man, and a girl reading. Let me caution you that seeing these figures or any other requires a bit of imagination.

The Moon: Bane of Deep Sky Objects (DSOs) 
Dark skies are needed for the best viewing of the fainter deep sky objects like star clusters, nebulae and galaxies. A bright Moon hinders observing these objects because it “white washes” the sky by scattering its light, preventing us from seeing the fainter celestial objects. Additionally, when the Moon is bright, our eyes cannot fully adapt to the darkness — and night vision is essential for viewing deep sky objects. So, unless you want to observe the Planets, Moon or Double Stars, you must work around the Moon’s schedule to view the deep sky objects. There is always a flurry of amateur activity on weekends closest to the New Moon in order to take advantage of a long dark night nestled against a day for sleeping in.  

Why don’t we have both a solar and lunar eclipse every month? Because the Moon’s orbit is tilted 5.1° to the Earth’s orbit. This usually places the Moon above or below the Sun and Earth’s shadow at Full Moon and New Moon.


The Moon appears to cycle through phases because it orbits the Earth. In reality, one-half of the Moon is always bathed in sunlight just like with the Earth. So, the phases represent the portion of the sunlit side of the Moon that we see from Earth. I have found that some individuals do not understand why the Moon has phases. I believe that this misunderstanding is partially because people get confused quickly with explanations involving geometry, which is the case with the Moon’s phases.

So, if you are one of these individuals having difficulty understanding the phases of the Moon, I don’t know if I can explain it here to your satisfaction, but I will try. You can partake in a “down-to-earth” example that mimics the phases of the Moon. On sunny days, when you are out and about walking the city streets, take a closer look at any smooth round pole that is bathed in sunlight.

One “side” or half of any such pole is always lit by sunlight (its day side) while the other half is always “dark” (its night side). On a smooth pole, and in bright sunlight, there is usually a fairly well-defined vertical line dividing the sunlit side from the dark side. Now, walk slowly around the pole, observing and noting the amount of “light” to “dark” that you see from different sides. This is the same effect that accounts for the phases of the Moon, except the Moon moves around us instead of us walking around it.

Observing the Moon through a Telescope

The prime time to observe the Moon is during its waxing and waning phases. Waxing means “adding on” and waning means “subtracting from.” The terminator, the “line” separating the lighted side from the dark side, is present when the Moon is waxing and waning. Craters appear their best (sharpest) when near the terminator because the contrast from the shadows makes them more pronounced. Magnifications from 40x to 250x are recommended. On a night when the atmosphere is steady and using higher magnifications, it can seem like you are flying right about the craters! 

The best views, that is to see more detail, of the Moons surface, is observing on or close to the terminator, the “line” that separates the day from the night side.

The Moon is disappointing to observe around Full Moon. During this time, the entire surface, along with most features, is “washed out.” However, at this time, the rays of craters are at their most pronounced. The crater Tycho’s rays stretch halfway across the hemisphere. 

Rays emanating from the southern crater Tycho, and the low contrast, “washed out” look from a full Moon. Tycho is a prominent crater in the southern part of the Moon and represents a more “recent” impact.

“Washed out” Full Moon

Want to observe features on the dark or night side of the Moon? Around New Moon, when the Moon is a thin crescent, the dark side of the Moon is slightly lit by reflected light from Earth called Earthshine. Some features on the dark side are visible in a telescope at this time. Try observing this night side—it is a pretty sight! See picture on this page.

Earthshine. This overexposed picture of a Waxing Crescent Moon brings out the dark side of the Moon which is softly lit by reflected Earth light. Try observing this shortly after New Moon. Yes, the overexposed crescent is what we actually see in the sky.

Need Filters to Observe Moon in a Telescope
Any telescope concentrates lots of light when focused on the Moon, which can easily strain your eye unless you use an eyepiece filter (these screw into threads at the bottom of the barrel) to reduce the intensity/fatigue. A common practice is to purchase two polarizing filters, A, which are rotated against each other to vary the amount of light passing through, B and C
Note: Some inexpensive eyepieces are not threaded to accept filters.

Major Moon Features

The most notable features on the Moon are its brighter cratered highlands called terrae and smoother darker plains known as maria. These and other features are described below.

Terminator. The border or “line” separating the lighted side from the dark side. The terminator is absent during Full Moon. Craters appear at their sharpest near the terminator. 
Craters. Huge bowl-like depressions on the Moon. Most of the craters on the Moon were formed from meteoroid or cometary impact that ended about 3.2 billion years ago. 
Terrae & Maria. Terms coined by Galileo meaning “highlands” and “seas” respectively. The lighter-colored terrae have the highest concentration of craters and are older and higher than the maria. The darker maria are smoother areas of the Moon and represent 16% of its surface. They are the result of impacts from large asteroids or comets creating fractures to the once molten interior, releasing dark, iron-rich, basalt lava, which flowed upward and outward to create the great plains. They average 500 to 600 feet thick. There are very few maria on the far side of the Moon. 

The Straight Wall or more formally, Rupes Recta, a fault line that is 70 miles long and 1,000 feet high. 

Rille. A long cliff or split in the maria, up to hundreds of miles or kilometers in length. Rilles can be seen in a telescope. They are the result of cracks, fractures or collapses in the maria. The most famous is the Straight Wall, which is very easy to see in any telescope (see picture on this page. 
Rays. Bright streaks that radiate from some craters. They represent lighter, reflective material, ejected during the formation of craters and are most pro-nounced around Full Moon. The crater Tycho has the longest rays, spanning one-quarter of the globe. It is estimated that rayed craters are less than one billion years old because the rays of older craters have been eroded by micrometeorites. 
Regolith. A fine grained “soil” that covers the surface of the Moon. Created from the bombardment of the surface by micrometeorites, the regolith varies in depth from 62 to 26 feet in the maria, and to a possible 49 feet in the highlands. The micrometeorites that bombard Earth burn up in the atmosphere.