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Astronomy for Beginners: Observing the Moon

Marco is a hobby astronomer and space enthusiast. He also enjoys learning and writing about the history of science.

observing-the-moon

Mankind has been fascinated by the stars since antiquity. The celestial sphere, besides inspiring mythology, was used by mariners for navigation and helped farmers track seasons. When ancient astronomers began to study the movements of celestial bodies, the oldest of sciences was established.

By contrast, in our modern society, people are often surprisingly ignorant about the stars. Yet looking up at the sky, identifying celestial objects, and comprehending their movements can be a truly awe-inspiring experience. For aspiring sky gazers, the Moon is a great place to start.

Our only natural satellite.

Our only natural satellite.

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: it takes the Moon 27.5 days to orbit the Earth once (relative to the background stars).

Although the true movement of the Moon is from west to east, it actually appears to be moving from east to west, like the Sun and the stars. This is the combined result of the Earth's rotation and the Moon's long revolution period.

As the Earth spins and its natural satellite slowly moves 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.

Furthermore, while rotating on its axis the Earth also orbits around the Sun. Because of this, a day on Earth lasts slightly longer than an exact 360-degree turn: the so-called sidereal day of just 23h 56min. Yet out of convenience, a day is measured in reference to the Sun: the solar day of 24h.

Likewise, the Moon cycle lasts slightly longer (29.5 days) than its orbital period (27.5 days), or about one month. The similarity of the terms 'month' and 'Moon' are not by coincidence but due to 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 that can be seen from a particular position on Earth (the phases don't have anything to do with the Earth's shadow as people sometimes think).

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

When the Moon is closer to the sun we see mostly the night side of the Moon (crescent and new phases). Conversely, when the Moon is farther away from the Sun we see mostly its 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. These correspond, respectively, to the 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.

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

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Eclipses

Unlike the natural satellites of other planets, the Moon orbits roughly in the ecliptic and not the celestial equator. With this, the Moon 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 two eclipses per month: one solar and one lunar, with each event separated by six months. Yet the Moon's orbital plane differs about 5 degrees relative to the ecliptic. Only where the two planes intersect (nodes) an eclipse is possible. For these events to occur 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 be perfectly stable in its orbital plane, eclipses would always occur on the same day. In reality, the plane of the Moon's orbit (though not its 5-degree tilt versus the ecliptic) does move westward in an 18.6-year cycle.

This cycle leads the nodes to move, a phenomenon that is known as the lunar precession. Because of this eclipses occur (roughly) in an 18-year cycle.

Solar Eclipse

Solar Eclipse

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 closely at the Moon's rim.

This is because the Moon's orbit is not exactly circular but slightly elliptical. Therefore the Moon is orbiting slightly faster when closer to Earth and slower when farther away. Thereby the rotational spin remains, on average, constant matching 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. Because of libration sometimes the North Pole is more visible, at other times the South Pole.

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

How the Moon Tricks Your Eye

The Moon has a diameter of 2,159 mi (3,475 km), but that corresponds only to about a one-half degree in angular size when 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 the Moon is near the horizon, it appears bigger than when high up in the sky. This illusion is simply due to the human brain perceiving the Moon as being larger when it's near the horizon, where it has other objects to set against it.

The full Moon near the horizon

The full Moon near the horizon

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 quarters when the sunlight falls in at an angle, the Moon looks spectacular due to the long shadows of its craters and mountains.

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

Sources

  • The Stargazer's Guide to the Night Sky, by Dr. Jason Lisle (Master Books, 2012)
  • The Astronomy Book, by Dr. Jonathan Henry (Master Books, 1999)
  • Destination: Moon, by astronaut James Irwin with Al Janssen (Master Books, 1989)
  • Wikipedia, Moon

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2017 Marco Pompili

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