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Moon phase

The moon has always fascinated people. Sometimes it shines in full and illuminates the entire night sky and sometimes only a small crescent can be seen. In addition, it provides tides and ocean currents.

The moon is not a light source, but a non-self-illuminating object, i.e. a passive light source. This means the moon only reflects the light that falls on it from the sun. If you look closely at the crescent moon, you can often see the side of the moon that is not illuminated by the sun. This is called the ashen moonlight. This comes from the earth, which in turn reflects the sunlight to the moon.

One speaks of the individual moon phases when the moon is illuminated differently by the sun during its cycle around the earth depending on its position. The phases of the moon are caused by the fact that the moon rotates around the earth and, depending on its position, a certain part of its surface reflects light towards the earth.

The moon is almost continuously illuminated by the sun. Since it does not shine itself, only its half illuminated by the sun is always bright. Depending on the position of the moon on its orbit around the earth, an earthly observer sees different amounts of this illuminated half.

A lunar phase cycle lasts approximately 29.5 days and includes new moon, waxing crescent, full moon and waning crescent.

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What are the phases of the moon?

During the entire lunar cycle, 4 or 8 lunar phases are distinguished. The 4 main phases of the moon are new moon, waxing moon, full moon and the waning moon. All moon phases are distinguished as follows:

New moon: In the lunar phase of the new moon the moon is between earth and sun (in conjunction). The side facing the earth is then completely in shadow and thus dark. (elongation = 0°; phase angle = 180°)

Waxing Moon: The waxing moon is the beginning of the lunar cycle. In this lunar phase the moon is illuminated from the right side in the northern hemisphere and from the left side in the southern hemisphere.

First quarter: The first quarter is when the moon has reached the first half phase and is 50% illuminated. (eastern elongation < 90°; phase angle > 90°)

Waxing crescent moon: The waxing crescent moon is more than 50% illuminated and is directly before the full moon phase. (astronomical: first quarter; easterly elongation ≈ 90°; phase angle = 90°)

Full moon: One speaks of the full moon when the earth lies between moon and sun (opposition). From the night side of the earth the moon is then fully illuminated. (elongation = 180°; phase angle = 0°)

Waning crescent moon: The waning crescent moon is also more than 50% illuminated, but the luminosity of the moon decreases with each day. (astronomical: last quarter; western elongation ≈ 90°; phase angle = 90°)

Last quarter: The second quarter is the second half phase, when the moon is again 50% illuminated in its cycle. (western elongation < 90°; phase angle > 90°)

Waning Moon: The waning moon initiates the end of the lunar cycle. Then only the left side of the moon is illuminated in the northern hemisphere and the right side in the southern hemisphere.

Rule of thumb: If the crescent moon opens a bracket, it is a waning moon. If it closes the bracket, it is a waxing moon. This only applies to the northern hemisphere.

Video file

The lunar phase depends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the sun. This animation (not to scale) looks down on Earth from the north pole of the ecliptic.

Effect of parallax

The Earth is at an angle of about two degrees as seen from the Moon. This means that an observer on Earth who sees the moon when it is near the eastern horizon will see it from an angle that differs by about 2 degrees from the line of sight of an observer who sees the moon on the western horizon. The Moon moves about 12 degrees per day in its orbit. Thus, if these observers were stationary, they would see the phases of the Moon at times that differ by about one-sixth of a day, or 4 hours.

In reality, however, the observers are on the surface of the spinning Earth, so someone who sees the Moon on the eastern horizon at a given time will perceive it on the western horizon about 12 hours later. This causes an oscillation in the apparent sequence of lunar phases. They appear to move more slowly when the moon is high in the sky than when it is below the horizon. The moon appears to move in a jerky fashion, and the phases do as well. The amplitude of this oscillation is never more than four hours, which is a small fraction of a month. It has no obvious effect on the appearance of the Moon. It does, however, interfere with accurate calculations of the phases of the moon.

Eclipses

You might expect that once a month, when the moon passes between the earth and the sun at new moon, its shadow falls on the earth and causes a solar eclipse, but this does not happen every month.

It is also not true that every full moon the Earth's shadow falls on the Moon and causes a lunar eclipse. Solar and lunar eclipses are not observed every month because the plane of the Moon's orbit around the Earth is tilted about 5° with respect to the plane of the Earth's orbit around the Sun (the ecliptic plane). Therefore, at new and full moon, the moon is usually north or south of a direct line through the earth and sun. Although an eclipse can only occur at new moon (Sun) or full moon (Moon), the Moon must also be very close to the intersection of the Earth's orbital plane around the Sun and the Moon's plane around the Earth (i.e., at one of its nodes). This happens about twice a year, so there are between four and seven eclipses in a calendar year. Most of these eclipses are partial eclipses; total eclipses of the Moon or Sun are rarer.

Eclipses - lunar nodes to rotate relative to the Earth

As the Earth revolves around the Sun, the approximate axial parallelism of the lunar orbital plane (which is tilted five degrees to the Earth's orbital plane) causes the lunar nodes to rotate relative to the Earth. This results in a solar eclipse occurring approximately every six months, with a solar eclipse occurring during the new moon phase and a lunar eclipse occurring during the full moon phase.

Waxing or waning?

When the moon is nearly full, it can be difficult to tell from its shape whether it is waxing or waning. The best clue is the position of the sun at moonrise. If both are visible, the moon is waxing, but if the sun has already set, the moon is waning.

Whether the moon is in a waxing or waning phase can be quickly determined by which side of the moon the shadow is on. If the shadow is on the right side, as it is today, we are in a waning phase. If the shadow is on the left side, then we are waxing and heading toward a full moon. An easy way to remember this is to rhyme light and right.

In order to remember on which side the curve is, here are some neat mnemonics.

A well-known example in English for the waxing moon is the spelling of the lowercase letter "b", which has a curve on the right side and thus means "beginning". For the waning moon, the "d" is used, which has its curve on the left side and therefore means "waning".

• ☽ = »b« = beginning (waxing)
• ☾ = »d« = descending (waning)

How to Tell if the Moon Is Waning or Waxing?

Another mnemonic describes the order of the waxing crescent → full moon → waning crescent, uses the shape of letters too, in this case the »D« and the »C« and creates the abbreviation »DOC«:

• ☽ = »D« = waxing moon
• O = »O« = full moon
• ☾ = »C« = waning moon

These mentioned mnemonics only work in this way in the northern hemisphere of the Earth. For the southern hemisphere you have to reverse the mnemonics (so to say »to turn them topsy-turvy«), because there, the curve of the moon is just the opposite for the observer.

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