Topic 3

• Who made the first measurements of the diameter of the Moon and Sun?

  Aristarchus
• What technique did Eratosthenes use to measure the circumference of the Earth?


  He used the shadow stick technique.
• What is a lunar eclipse?

  The Earth passing directly between the Sun and Moon casting a shadow on the Moon.
• What are the four contacts during a lunar eclipse?

  1. 1st umbral contact: Moon touches the Earth’s shadow
  2. 2nd umbral contact: Moon fully enters the shadow
  3. 3rd umbral contact: Moon starts to leave the shadow
  4. 4th umbral contact: Moon is completely out of the shadow
• When can a lunar eclipse occur?

  A lunar eclipse can only happen during a full Moon.
• What is a neap tide?

  When the Sun, Moon, and Earth are at right angles, high tide is less high and low tide is less low.
• What causes precession in the Earth's axis?

  Precession is caused by the wobble in the Earth’s axis.
• How long does one full wobble of the Earth's axis take?

  One full wobble takes 26,000 years.
• What is a solar eclipse?

  The Moon passing directly in front of the Sun, creating a shadow on the Earth.
• What are the four contacts during a solar eclipse?

  1. 1st umbral contact: Moon touches the edge of the Sun
  2. 2nd umbral contact: Moon completely covers the Sun
  3. 3rd umbral contact: Sun starts to reappear
  4. 4th umbral contact: Moon leaves the Sun’s disc entirely
• When can a solar eclipse occur?

  A solar eclipse can only happen during a new Moon.
• What is a spring tide?

  When the Sun, Moon, and Earth are aligned, high tide is extra high and low tide is extra low.
• What do tidal forces arise from?

  The inverse square nature of the force of gravitation
• How does the gravitational pull vary on a body due to tidal forces?

  The pull of gravity is greatest on the side nearest the attracting body and weakest on the opposite side
• What effect does the difference in gravitational pull have on a body?

  It causes the body to elongate slightly in the direction of the attracting body, creating tidal bulges
• What are the main effects of tidal forces on Earth and its moons?

  • High and low tides on Earth
  • Flexing and internal heating of some moons (e.g., Io and Enceladus)
  • Creation of ring systems around gas giants
  • Synchronous rotation of large planetary moons
• How do tidal forces contribute to high and low tides on Earth?

  Tidal forces cause Earth to spin into and out of the bulges of water on opposite sides
• What is the role of tidal forces in the volcanic activity of moons like Io?

  Tidal forces cause flexing and internal heating, allowing volcanic activity
• How do tidal forces affect the presence of liquid oceans beneath icy crusts on moons like Enceladus?

  Tidal forces create internal heating that maintains liquid oceans beneath icy crusts
• What is the relationship between tidal forces and the creation of ring systems around gas giants?

  Tidal forces can rip apart moons that are too close to a planet, forming rings
• What is the significance of the graph showing the force of gravity on a unit mass (g) with distance from a massive planet?

  The graph illustrates how gravitational force decreases with distance, following the inverse square law

  
• What happens to a large moon placed close to a planet in terms of tidal forces?

  The tidal forces on it will be much greater than if it were placed further away
• What is ∆g in the context of tidal forces on moons?

  ∆g is the difference in gravitational force on the near side of the moon and the far side
• What can happen to a moon that experiences large tidal forces greater than the elastic forces holding it together?

  It may be ripped apart to form smaller bodies, such as rings
• How does the distance of a moon from a planet affect the likelihood of it being damaged by tidal forces?

  A moon further away experiences much smaller ∆g, which is insufficient to cause permanent damage
• What is the Roche Limit (R) in relation to moons and tidal forces?

  The Roche Limit is the minimum distance at which a sizeable moon can form without being torn apart by tidal forces
• What is the rule-of-thumb value for the Roche Limit?

  R ~ 2.5 times the planet’s radius
• How do small moons differ in their susceptibility to tidal forces compared to larger moons?

  Small moons may not encounter significant ∆g forces even when close to the planet
• What factors influence the likelihood of moons being broken up by tidal forces?

  • Size of the moon
  • Distance from the planet
  • Strength of tidal forces (∆g)
  • Elastic forces holding the moon together
• What does the graphic summarizing the likelihoods for moons of different sizes and distances indicate?

  • Large moons: highly probable to be broken up if close
  • Small moons: highly improbable to be broken up if far
  • Small moons: possible to be broken up if close

  
• What is an eclipse?

  An eclipse is when the shadow of a celestial body obscures the light of another object in space.
• What are the two main types of eclipses?

  • Solar eclipse: Sun obscured by the Moon
  • Lunar eclipse: Moon obscured by Earth's shadow
• Why do observers on Earth see the Sun and Moon as roughly the same size?

  Because the Sun is over 400 times larger than the Moon yet also over 400 times further away.
• What is the maximum duration of a total solar eclipse?

  7 minutes, 30 seconds
• What is the maximum duration of an annular solar eclipse?

  12 minutes, 30 seconds
• What is the maximum duration of a total lunar eclipse?

  1 hour, 40 minutes
• When does a solar eclipse occur?

  A solar eclipse occurs during a new Moon when the Moon is nearest to the Sun.
• What happens during a total solar eclipse?

  The Moon's disc completely covers the disc of the Sun.
• Why can total solar eclipses only last a maximum of 7 minutes 30 seconds?

  Due to the speed of the Moon orbiting Earth and the rotation of Earth.
• What is the 'line of totality' during a solar eclipse?

  It is the line on Earth's surface where the shadow is focused during totality.