Around the Sky in 28 Days – Day 0

The Moon is usually the most obvious object in the night sky. Because of this, it can act as the perfect starting point to highlight other sky attractions. This lunar month I will publish daily posts focusing on interesting objects near the Moon.

The Moon takes 27.3 days to orbit the Earth, what we call its orbital period. In those ~27 days the Earth has moved in its own orbit around the Sun. As a result, the Sun has moved on the sky and it takes the Moon a few more days to catch up with the Sun in its new position. The time it takes the Moon to catch up with the Sun again is called its synodic period and is 29.5 days long. It is this synodic period (you can think of it as the time between New Moons, or Full Moons) that is the basis of our calendar months.

This month New Moon occurs today, September 8, at 10:30 UT. At that time the Moon is located in the same direction as the Sun. Due to the Moon’s slightly inclined orbit (5.1°) it often passes to the north or south of the Sun. About twice a year it will pass in front of the Sun causing a solar eclipse for those lucky enough to be in the right area to see it. Regardless, the Moon is not visible at New Moon to visual observers because the glare of the Sun is too bright and very little of the Moon is illuminated by sunlight because we are seeing its dark side from Earth.

Tomorrow starts Day 1 – [Why you probably won’t see the Moon a day after New Moon in September even though it is an easy sight in March (otherwise known as a discussion of the tilt of the ecliptic).]