The Phases of the Moon

Everyone is familiar with the Moon. The Moon is so familiar to us that we often ignore it completely but the Moon is the most visible object in the night sky. There is a tremendous amount of detail that can be seen on the Moon through a telescope. As the Moon goes through its monthly cycle it changes its shape and size as seen from Earth. I try to do the Sidewalk Astronomy in Lynbrook sessions when the Moon is visible in the early evening. People seeing the Moon for the first time through a telescope are often amazed.

Looking at the Moon through a telescope, you can see craters and mountains. Looking at the Full Moon is not really the best time to view it. The brightness of the Sun’s light reflecting off the Moon during a Full Moon washes out a lot of detail. Looking at a Full Moon through a telescope requires a filter to dim the light a bit. It is best to view the Moon before and after Full Moon. The shadows created during these phases make the lunar features much more visible.

The line on the Moon that separates light from darkness is called the terminator. It moves across the Moon as it goes through its phases. The terminator is a good spot to see detail on the Moon. One half of the Moon is always illuminated by the Sun. The Moon goes through phases as we see the Moon from different angles during its orbit around the Earth. During these phases we are seeing various portions of the illuminated and dark surfaces of the Moon.

The Moon’s phases are as follows:

New Moon – the New Moon is not visible to us. The side of the Moon that is not facing Earth is lit up by the Sun. The side of the Moon in darkness (facing away from the Sun) is facing the Earth, so we often see no Moon at all. The Moon is between the Earth and the Sun. Sometimes we can see a darkened portion of the Moon due to light reflecting off the Earth and onto the dark side of the Moon. You would need to be in a very dark location to see this.

Waxing Crescent Moon – waxing means increasing in size, so the waxing Crescent Moon is “growing” - moving from New Moon to First Quarter Moon.

First Quarter Moon – this is half a Full Moon but is actually one quarter of the Moon’s surface.  This occurs 7 days after New Moon.

Waxing Gibbous Moon – moving from First Quarter to Full.

Full Moon – we are seeing the entire illuminated side of the Moon. This happens when the Moon is on the opposite side of earth from the Sun. So the Full Moon rises at sundown. This occurs 14 days after New Moon.

Waning Gibbous Moon – waning means decreasing in size, so the waning Gibbous Moon is “shrinking” – moving from Full to Last Quarter.

Last Quarter Moon – again this is half the Full Moon but this time it is the other half of the Full Moon seen at First Quarter. This occurs 21 days after New Moon. The Last Quarter Moon is also known as the Third Quarter Moon.

New Moon – completing the cycle and again not visible from Earth. This is the 28th day since the last New Moon.

The waxing Moon rises during daylight while the waning Moon rises during the night. The Moon is visible just as often during the day as it is at night but we tend to notice it much more at night. This makes sense as the moon provides significant light during the night but is barely recognizable against the daytime blue sky.

For More info:

Moon Phases Calendar:  http://www.moonconnection.com/moon_phases_calendar.phtml

Understanding the Moon’s Phases:  http://www.moonconnection.com/moon_phases.phtml