The Cycles of the Moon

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If the sun and the moon have the same angular diameter on the sky, does that mean that the sun and the moon actually have about the same size?

Yes.

No because the sun is much farther away, but also much larger than the moon.

No, because the sun is much farther away, but also much smaller than the moon.

No because the moon is much farther away, but also much larger than the sun.

No, because the moon is much farther away, but also much smaller than the sun.

Slide 24

Earth’s and Moon’s orbits are slightly elliptical:

Sun

Earth

Moon

(Eccentricities greatly exaggerated!)

Perihelion = position closest to the sun

Aphelion = position furthest away from the sun

Perigee = position closest to Earth

Apogee = position furthest away from Earth

Slide 25

What do you expect to see if at the time of a solar eclipse the moon is near apogee, and the Earth is near perihelion?

A regular total solar eclipse.

No solar eclipse at all.

A partial solar eclipse with a crescent appearance.

A partial solar eclipse with a ring-like appearance of un-occulted parts of the sun.

A lunar eclipse.

Slide 26

Annular Solar Eclipses

The angular sizes of the Moon and the Sun vary, depending on their distance from Earth.

When the Earth is near perihelion, and the Moon is near apogee, we see an annular solar eclipse.

Perigee

Apogee

Perihelion

Aphelion

Slide 27

Almost total, annular eclipse of May 3, 1984

Slide 28

If the moon was orbiting around the Earth exactly in the plane of the ecliptic, …

lunar and solar eclipses would occur once every day.

lunar and solar eclipses would occur once a week.

lunar and solar eclipses would occur once a month.

lunar and solar eclipses would occur once a year.

lunar and solar eclipses would never occur.

Slide 29

Conditions for Eclipses (I)

The Moon’s orbit is inclined against the ecliptic by ~ 5.

A solar eclipse can only occur if the Moon passes a node near New Moon.

A lunar eclipse can only occur if the Moon passes a node near Full Moon.

Slide 3

Conditions for Eclipses (II)

Eclipses occur in a cyclic pattern.

→ Saros cycle: 18 years, 11 days, 8 hours

Slide 31