Astronomy for Beginners: Observing the Moon

Updated on December 18, 2017

Mankind has been fascinated by the stars since the dawn of history. As ancient astronomers tried to understand the movements of celestial bodies, one of the oldest of sciences was developed. But today, people in our highly educated society are often surprisingly ignorant about the stars. Looking up at the sky, identifying celestial objects, and having an idea about what's going on above can be such an awe-inspiring experience. For aspiring sky gazers, the moon is a great place to start.

Our only natural satellite.
Our only natural satellite. | Source

The moon orbits the Earth in the same direction the Earth spins (i.e. counterclockwise looking down from the North Pole), albeit at a much slower rate: relative to the background stars it takes the moon 27.5 days to orbit the Earth once.

Although the true movement of the moon is from West to East, due to its long revolution period and the Earth's much faster rotation, the moon actually appears to be moving east to west, just like the sun and stars. As the Earth spins and its natural satellite slowly moves on along its orbit, the moon rises about 50 min later every morning and appears about 12 degrees farther east at the same time of the following day.

As the Earth also orbits around the sun while rotating on its axis, a day on Earth lasts slightly longer than an exact 360 degree turn, the so-called sidereal day of just about 23h 56min. Yet out of convenience a day is measured in reference to the sun: the solar day of 24h. For the same reason the moon cycle lasts slightly longer than the orbital period of the moon: 29,3 days (instead of 27,5 days) or about one month. The terms 'month' and 'moon' indeed have a common etymological origin.

The Moon's Phases

Viewed from Earth the moon goes through phases. These correspond to the part of day side of the moon one can see from a particular position on Earth (the phases don't have anything to do with the Earth's shadow as people sometimes think).

As the whole cycle lasts about one month, it takes the moon roughly one week to go through one-fourth of it: if today is new moon, in one week the moon will be in its first quarter, in two weeks there will be full moon, in three weeks it will be in its third quarter and then after four weeks reach new moon again.

When the moon is closer to the sun we see mostly the night side of the moon (crescent and new phases), with the moon farther away from the sun mostly the day side (gibbous and full phases). At a 90-degree angle we see a vertical line dividing the moon in half day and half night side (first and third quarter phases).

A waxing moon means the percentage of day side in increasing, while during a waning moon the percentage of dayside is decreasing. If you live in the northern hemisphere and the moon is illuminated on the left side it is waning, when lit on the right side the moon is waxing. The opposite is true for the southern hemisphere, while near the equator the sickle is at the bottom.

 
 
New Moon
0 hrs
Waxing Crescent
TRAILS sun by 3 hrs
First Quarter
TRAILS sun by 6 hrs
Waxing Gibbous
TRAILS sun by 9 hrs
Full Moon
TRAILS or LEADS sun by 12 hrs
Waning Gibbous
LEADS sun by 9 hrs
Third Quarter
LEADS sun by 6 hrs
Waning Crescent
LEADS sun by 3 hrs
New Moon
0 hrs
Source

Eclipses

Unlike the natural satellites of other planets the moon orbits (roughly) in the ecliptic and not the celestial equator. It therefore follows the path of the sun through the sky, albeit thirteen times as fast. This helps to locate the moon in the sky, especially if you know in which phase it currently is.

Seasonally the moon behaves the opposite of the sun. While the sun is high in the sky in summer and low in winter, the moon is high in winter and low in summer.

If the moon were to orbit the sun exactly in the ecliptic we would have to eclipses per month, one solar and one lunar, separated by six months. Yet the moon's orbital plane differs about 5 degree relative to the ecliptic. Only where the two planes intersect (nodes) an eclipse is possible, but the sun and the moon need to cross the nodes at the same time. If they cross opposite nodes at the same time, the Earth's shadow falls on the moon and a lunar eclipse occurs. Conversely, if they cross the same node simultaneously, the moon's shadow falls on the Earth and a (much more spectacular) solar eclipse takes place. Potentially there are only two times a year, separated by six months, that an eclipse is possible, that is, whenever the sun crosses the nodes that intersect the ecliptic and the plane of the lunar orbit.

Would the moon remain stable in the same plane, eclipses would always occur on the same day. In realty the plane of the moon's orbit (though not its 5 degree tilt versus the ecliptic) does move westward in a 18.6 year cycle. The nodes therefore move, a phenomenon known as the lunar precession. Eclipses therefore occur (roughly) in an 18-year cycle.

Solar Eclipse
Solar Eclipse | Source

Libration

The moon orbits at the same rate it revolves around the Earth. This is why we see always the same side of the moon. Yet keen observers will notice that this is not exactly true, especially when looking close at the moon's rim.

The reason is that the moon's orbit is not exactly circular, but slightly elliptical. Therefore the moon is orbiting slightly faster when closer to the Earth and slower when farther away, while the rotational spin is remains constant and, on average, matches the moon's orbital period. This phenomenon is dubbed the east-west libration.

There is also a north-south libration, although less noticeable. The latter is due to the fact that the moon's rotational axis doesn't exactly match its orbital plane, so that sometimes the North Pole is more visible, at other times the South Pole.

Due to libration, over time it is possible to see of up to 59% of the moon's surface from Earth (instead of a bare half).

How the Moon Tricks Your Eye

The moon has a diameter of over 3,000 km, but that is only about one-half degree in angular size, viewed from Earth. You therefore can easily block the moon with your finger at arm's length. That, of course, is true for any position of the moon in the sky. Yet when near the horizon, the moon appears bigger than when it is high up in the sky. This illusion is simply due to our brain perceiving the moon as being larger when it's near the horizon, as then it has other objects to set against it.

The full moon near the horizon
The full moon near the horizon | Source

The moon is a great target for beginning stargazers and experts alike. Observing the diverse phases will give you a feel about the basic movements of celestial bodies. Binoculars will suffice to learn the diverse Maria of lunar geography. On the other hand, the nearness of the moon makes it an excellent object for surface observation for astronomers equipped with more advanced optics. Especially during the first and third quarter the moon looks spectacular due to the long shadows of its craters and mountains, as then the sunlight falls in at an angle. Because of libration the moon will also look slightly different night after night. You will never look at exactly the same moon twice.

Earth Rising (Apollo 8)

View the other way round
View the other way round | Source

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© 2017 Marco

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