Motion of the Moon:
Phases & Eclipses
Motion and Phases of the
- Orbit: The Moon revolves the
Earth with a prograde orbit, with period (sidereal month) 27.3
days [it covers 12°/day, almost 1 km/sec], tilted 5°;
It is not exactly a circle [the distance can be measured to within
a few meters with radio waves], so the Earth-Moon distance varies
by about 13% over one orbit [it also increases by about 4 cm/year!].
- Rotation: The Moon shows always the
same side to us because it completes exactly one rotation per revolution;
We will see that there is a reason.
- Phases of the Moon: The main
ones are new, quarter (waxing, waning), full Moon; The Sun always
illuminates half of the Moon, so why do we see different phases?
- Relationship with time of the day:
Because the phase of the Moon depends on its position along its
orbit, the times of day at which can we see the Moon in each
phase varies. What is the relationship?
- Sidereal vs lunar (synodic) month:
The cycle of phases repeats every 29.5 days; Why not 27.3 days,
which is the orbital period?
- What are they? The concepts
of umbra and penumbra of the Earth on the Moon.
- Types: Can be penumbral, partial
or total; Where can they be seen? Totality may last a long time.
- Appearance: During a total lunar
eclipse, the Moon is reddish, rather than totally dark; Why?
- Types: Partial, total and annular;
Why do they happen? What makes them sometimes total and sometimes
- Visibility: Only from a small
area, and totality is only a few minutes long! But one can then
see features of the Sun's atmosphere (and the Moon).
- How often? Favorable eclipse
seasons occur when the orbit's nodes cross the ecliptic, about
twice a year; Exact dates are very difficult to predict [the
18-year Saros cycle]; How has science enabled us to predict them
Curiosities: What is a "syzygy"? Which of
the phenomena above is an example?
- Earthshine: Sunshine reflected
by the Earth's surface to the Moon.
- Moon illusion: Is the Moon's
disk really larger when it is close to the horizon?
- Moon halo: Why does the Moon
appear to have a halo sometimes? Because of effects caused by
the Earth's atmosphere.
- Eclipses elsewhere? Star occultations;
Earth-Sun from space; stars eclipsing each other, extrasolar
But no other planet in the Solar System has the kind of eclipses we have...
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 aug 2012