Phases of the MoonAstronomy

Phases of the Moon

The Moon is always half illuminated as it orbits Earth.

Observers on Earth will see only a portion of the lit side depending on the positioning of the Moon.

Phases are NOT because of shadows.

Phases of the Moon

It takes the Moon 27.3 days to orbit and line back up with the stars. (sidereal period = 27.3 days)

Since the Earth is moving in space too, it takes 29.5 days for the Moon to go from New Moon to New Moon (synodic period = 29.5 days)

Sideareal vs.Synodic

sidereal period = 27.3 days

synodic period = 29.5 days

Rotation of Moon

The Moon slowly rotates as it orbits, so the same side always faces the Earth (“tidally locked”).

There is always a dark side of the Moon, but it’s not the same side that’s always dark.

For instance, at New Moon, the far side of the Moon is lit, but at Full Moon, the far side is dark.

Watch the line on the Moon.

Notice it always faces the Earth.

Orbit of Moon

The Moon’s orbit is not a perfect circle.

The Moon can be slightly larger when closer and slightly smaller when farther (but you probably wouldn’t really see it with your naked eye).

Example

The diagram below shows the relative positions of the Earth, Moon, and Sun for a 1-month period. Which diagram best represents the appearance of the Moon at position P when viewed from the Earth?

Waning crescent

Example

The phases of the Moon are caused by the

a) Sun's varying distance from the Moon

b) Moon's revolution around the Earth

c) Moon's varying distance from the Earth

d) Earth's revolution around the Sun

Example

Use the diagram.

At position 2, the Moon is in which phase?

a) New Moon

b) Waxing crescent

c) 1st quarter

d) Waning crescent

Example

Use the diagram.

At position 5, the Moon is in which phase?

a) New Moon

b) Full Moon

c) 1st quarter

d) Waxing Gibbous

Example

Use the diagram.

Shade in the circle to illustrate the Moon’s phase at position 7.

Name the phase at 7.

3rd Quarter Moon

Example

What is the approximate length of time the Moon takes to travel from position A to position C?

a) 1 day

b) 15 days

c) 30 days

d) 365 days

New moon to New

Moon takes about 30

days, so A to C takes

about 15 days or so.