Astronomy paper 1 topic 2

• Most people are surprised when they learn that out of the Earth, Moon and Sun, it is the Sun that is the most spherical in shape
• The Earth's polar diameter is smaller than its equatorial diameter by 42 km, making its mean diameter 13000 km
• For the Moon, the difference is only 4 km compared with its mean diameter of 3500 km. So like the Earth, the shape of the Moon is also an oblate spheroid
• A 10 km difference for the Sun is insignificant compared with its mean diameter of 1.4 million km; the Sun is almost perfectly spherical
• Even though the full disc of the Moon subtends an angle of only 0.5" at the human eye, on clear nights and with perfect seeing conditions, many features of the Moon are discernable without resorting to the use of optical aids
• Lunar surface features


  • Large dark-grey, relatively smooth seas (maria) of volcanic basalt rock
  • Lighter-grey, mountainous, highly cratered highlands (terrae) of igneous rock called anorthosite
• In contrast, the Moon's far side is almost devoid of maria
• Formation of lunar craters

  1. Meteoroids striking the lunar surface

  2. Shockwave compressing the surface material to leave a large cavity

  3. Rebound splattered material (ejecta) out in all directions, creating bright streaks (rays)
• The Moon revolves around the Earth once in 27.3 days; this is also the time taken for the Moon to rotate on its axis by 360°
• Most large moons of the giant planets also show synchronous rotation, arising from internal tidal gravitational forces slowing down the moons' periods of rotation until these become tidally locked to their orbital periods
• Sidereal month


  The Moon's orbital period of 27.3 days
• Synodic or solar month


  The time interval from one full Moon to the next, which is 29.5 days
• Although the Moon is in a synchronous orbit and only shows us its near side, over a period of time it is actually possible to observe up to 59% of the Moon's surface from Earth due to various kinds of lunar libration
• Causes of lunar libration

  • The Moon's equator is inclined to the plane of its orbit around the Earth by 1.5%
  • The plane of the Moon's orbit is inclined at 5.1° to the ecliptic
  • The Moon's varying speed in its elliptical orbit around the Earth
• Libration allows observers to see slightly around the Moon's eastern and western limbs at moonrise and moonset
• In accordance with Kepler's second law, the Moon takes the same time to travel in its orbit from perigee to a point as it does from that point to apogee, but its rotation rate remains constant, allowing observers to see a little more around its eastern and western limbs