Phases, Eclipses and Tides

The Moon is in motion as it rotates on its axis and revolves around the Earth. The changing relative positions of the Sun, Earth and Moon cause moon phases, eclipses and tides.

The changing appearances of the Moon as it makes one complete revolution around the Earth are known as moon phases. The positions of the Sun, Earth and Moon determine the portion of the Moon that is lit by sunlight. Sometimes the entire face of the Moon is lit, and at other times only part of the Moon is lit and the rest is in shadow.

A new moon occurs when the sunlit portion of the Moon is facing away from the Earth. The Moon is waxing when the sunlit portion is getting larger. A full moon is seen when the side of the Moon facing the Earth is entirely sunlit. The Moon is waning when the lit portion is getting smaller. Crescent and gibbous are shapes of the Moon that we see during a month.

When one celestial body comes between the Sun and another celestial body, a shadow is cast on the second celestial body. This phenomenon is called an eclipse. When the Moon's shadow hits the Earth, a solar eclipse occurs. When the Earth's shadow hits the Moon, a lunar eclipse occurs.

A solar eclipse occurs when the Moon passes directly between the Earth and the Sun, and sunlight is blocked from hitting the Earth. A total solar eclipse will only occur during a new moon. The sky turns dark and the temperature drops during the eclipse. The Sun's outer atmosphere called the corona also becomes visible.

During a solar eclipse, the Moon will cast the darkest part of its shadow, called it's umbra, on a small portion of the Earth. Only these locations will experience a total solar eclipse. Other locations within the Moon's penumbra, a wider shadow, will experience a partial solar eclipse.

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, and sunlight is blocked from hitting the Moon. A total lunar eclipse only occurs when the Moon is full. The entire Moon falls within the Earth's dark shadow, or umbra. The completely darkened Moon can be seen at all locations on Earth where the Moon is visible.

One might expect that lunar and solar eclipses would occur every month as the Moon orbits the Earth. This does not happen because the Moon's orbit is actually tilted by a little more than five degrees relative to the Earth's orbit around the Sun. It is rare that the Moon is in the Earth's shadow during a full moon or between the Earth and Sun during a new moon.

Changing water levels of the oceans are known as tides. Both the Sun and Moon influence tides. The gravitational pull of the Moon creates bulges in the Earth that impact the timing and height of tides. When ocean water is pulled by the Moon's gravitational force to create high tides in one area, water leaves other areas and creates low tides.

When the Sun, Earth and Moon line up with each other, the gravitational pull on the oceans is at its maximum. This alignment causes unusually high tides called spring tides. These tides occur twice a month during the new moon and the full moon.

When the Sun and Moon are at 90 degrees to one another, the total gravitational pull on the oceans is at its minimum, and the high tide is called a neap tide. These tides have the least difference between consecutive high and low tides. Neap tides occur twice a month during the first and third quarter moon phases.