Topic 3 - The Earth-Moon-Sun system

• Why do the sun and moon appear to be the same size?

  the sun is 400x further away from us than the moon

  the moon is 400x smaller than the sun
• what is the axial tilt of earth?

  23.5 degrees
• what caused the earth to become an oblate spheroid?

   rotation of the early molten Earth.
• what is the mean diameter of the sun?

  1.4 x10^6 km
• what is precession?


  Change in orientation of the rotational axis of a rotating body.
• what is precession caused by?

  Gravitational forces of the sun, moon and planets
• one full rotation of the earths axis takes?

  26,000 years
• currently the north pole of the sky lies near..?

  polaris
• in 2500 BC the north pole of the sky lied near thuban
• in 12,000 yrs what will be the pole star?

  vega
• where are the equinoxes?

  the points where the ecliptic crosses the celestial equator
• the vernal equinox marked by the first point of...?


  aries
• why has the vernal equinox moved to the constellation of pisces?

  Precession.
• which direction are the equinoxes slowly drifting?

  westwards
• what has precession caused?

  the celestial pole star to move

  the sun to rise at a different time 
• what is archeoastronomy?


  study of how ancient civilisations have used celestial phenomena
• how has precession affected archeoastronomy?

  it has caused the misalignment of ancient monuments and temples
• tides are the rise and fall of sea levels caused by the combined effect of the gravitational forces of the moon and sun, and the rotation of the earth
• when the moon passes in front of the sun, what can be seen on earth?

  a solar eclipse
• where can a total solar eclipse be seen?

  for observers in the dark shadow (umbra) on earth
• which layer of the sun can be seen during a total solar eclipse?


  corona
• The Sun is 400 times further away from us than the Moon but is 400 times larger and so they appear to be the same size
• at what stage of the moon does a solar eclipse occur?

  new moon
• what is the position of the moon during a a solar eclipse?

  Between Earth and the Sun
• why don't solar eclipses happen every new moon?

  Tilt of moon's orbit
• when are partial solar eclipses observed?

  when the observer is located in the penumbra
• what causes an annular solar eclipse?

  when the moon is furthest away from the earth (apogee), and so the umbra does not touch the earth
• what appearance does an annular solar eclipse have for an observer on earth?

  the outer ring of the sun is visible because the moon is too far away to completely cover the sun's disc
• what are umbral contacts?

  the progress of a total eclipse marked by 4 umbral contact (U1-U4), between the discs of the moon and the sun
• what happens at first contact (U1)?

  the edge of the umbral shadow falls on earth, total eclipse begins
• what is noticed on earth during first contact?

  a partial eclipse (moon starts to overlap that sun)

   sky will darken 

  temperature drops
• what happens at second contact (U2)?

  the entire umbral shadow first falls on earth, totality begins
• what can be noticed during second contact on earth?

  baileys beads (before totality)

  sky becomes black (during totality)

  suns corona is visible (during Totality)
• what are bailey's beads?

  just before totality bright spots of sunlight caused by the suns rays shining through valleys on the moon are visible
• what is the diamond ring effect?

  when only one of baileys beads are visible
• what happens at third contact (U3)?

  the umbra moves along the line of totality, and the moon starts to move away
• can baileys beads be observed during third contact?

  yes
• what happens during fourth contact (U4)?

  The edge of the umbral shadow last falls on earth, total eclipse ends
• at what phase of the moon does a lunar eclipse occur?

  full moon
• when the earth is directly aligned between the moon and the sun, and the shadow of the earth falls onto the moon. what occur?

  Lunar eclipse