The Moon

Phases of the MoonChapter 3 - Part 2

Objectives

• To be able to describe the phases of the Moon and why they occur.

Phases of the Moon

The Moon revolves counter clockwise around the Earth.

The Moon always shows us the “same face.”

Due to the stability of the Moon’s revolutions (and thus phases) the Moon is useful to migrating birds.

Many birds seem to have evolved over time to use the Moon as a navigational cue.

Phases of the Moon

Phases of the Moon

• As the Moon orbits Earth, it rotates to keep the same side facing Earth.

• Therefore, you always see the same features on the Moon.

• You never see the far side of the Moon.– The Apollo astronauts were the first and still the

only humans to directly see the far side of the Moon.

Phases of the Moon

• Sunlight always illuminates half of the Moon.• The “phases” are a product of our seeing

different amounts of the sunlit side. • When there is a new Moon, sunlight

illuminates the far side of the Moon.– The part we see is completely dark.

• At full Moon, the side we see is full lit.• The amount of Moon we see is due to where

it is in its orbit around Earth.

Phases of the Moon

• All parts of the Moon experience day and night.– Thus there is no such thing as the “dark side of the

Moon.”In the first diagram you can see that the new Moon is close to the sun and the full Moon is opposite the sun. Time of day depends on the observer’s location on Earth.

Phases of the Moon

Waxing MoonThe Moon grows “fatter” from new to full.

WaningThe Moon shrinks from full to new.

http://www.moonconnection.com/moon-october-2014.phtml

Phases of the Moon

• Sidereal Period – the Moon orbits eastward around Earth every 27.32 days. – This is how long the Moon takes to circle the sky

once – returning to where it started. • Synodic Period – a complete cycle of lunar

phases (takes 29.53 days). – The month is based on this period.

This graphic is from the Astronomy textbook, page 35, and is an explanation of the difference between sidereal and synodic periods.

At the bottom of the graphic is a chart of “Times of Moonrise and Moonset.

The Ecliptic

The ecliptic marks the path of the sun. It’s the projection of Earth’s orbit onto the sky (celestial orbit).

Questions to Consider

• About how many days must elapse between first-quarter moon and third-quarter moon?

• How much of the Moon is always illuminated by the sun?

• Is there really a dark side of the Moon? Why does it seem, to us, that there is a dark side?

• How are the sun and Moon interacting when there is a full Moon? When there is a new Moon?