Motion of the Moon:
Phases & Eclipses

Motion and Phases of the Moon

  • 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?

  Lunar Eclipses

  • 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?

Solar Eclipses

  • Types: Partial, total and annular; Why do they happen? What makes them sometimes total and sometimes annular?
  • 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 accurately?

  Other Phenomena

  • 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 planets... But no other planet in the Solar System has the kind of eclipses we have...

Curiosities: What is a "syzygy"? Which of the phenomena above is an example?

page by luca bombelli <bombelli at>, modified 29 aug 2012