3 The Earth-Moon-Sun System

Question 1

How do tidal forces occur?

Answer 1

The inverse square nature of the force causes the gravitational pull of the larger object on the smaller object’s nearer side to be greater than that of its far side. This is equivalent to two equal and opposite forces acting on the near and far sides that tend to stretch and elongate the smaller object.
The smaller object also causes a similar, but weaker force on the larger object

Question 2

How do tidal forces affect the Earth-Moon system?

Answer 2

The Moon exerts tidal forces onto the Earth causing tidal bulges in the oceans on the parts of the Earth facing towards and away from the Moon

Question 3

How many times does the coast experience high or low tides?

Answer 3

There are two daily high and low tides

Question 4

What are Lunar tides?

Answer 4

Tides caused by the Moon

Question 5

What are Solar tides?

Answer 5

Tides caused by the Sun

Question 6

What are Neap tides?

Answer 6

‘Low’ high tides

Question 7

What are spring tides?

Answer 7

‘High’ high tides

Question 8

When can spring tides occur?

Answer 8

Spring tides can only occur when the moon is in the full or new phase

Question 9

Why do spring tides occur?

Answer 9

The positions of the Solar and Lunar tides add up.

Question 10

When can neap tides occur?

Answer 10

Neap tides can only occur during quarter moons

Question 11

Why do neap tides occur?

Answer 11

Neap tides occur as the solar tide adds to the lunar low tide, which partially cancels it out

Question 12

What is another consequence of tidal forces?

Answer 12

Precession

Question 13

What is precession?

Answer 13

Precession is the change in the orientation of the rotational axis of a rotating body

Question 14

What is the evidence of precession?

Answer 14

• The changing ‘Pole Star’
• The changing locations of the equinox: these are drifting westwards. For example, the First Point of Aries that is used to mark the spring equinox but this is now in Pisces
• The misalignment of ancient monuments and temples

Question 15

What is a total solar eclipse?

Answer 15

When the Moon passes directly in front of the Sun , it obscures light for observers and produces a dark shadow.

Question 16

What is the Umbra?

Answer 16

The area during a Solar eclipse where the Sun is fully covered by the Moon

Question 17

What is the Penumbra?

Answer 17

The area of observation where the Sun is partially covered

Question 18

What are Baily’s Beads?

Answer 18

Baily’s beads are when the Sun’s rays shine through the valleys of the Moon and are visible

Question 19

What is the Diamond Ring effect?

Answer 19

The Diamond Ring effect is when only one Baily bead is visible

Question 20

What is the First Umbral Contact for a Solar Eclipse?

Answer 20

When the Moon first touches the Sun

Question 21

What is the Second Umbral Contact for a Solar Eclipse?

Answer 21

The Moon is just about to fully cover the Sun

Question 22

What is the Third Umbral Contact for a Solar Eclipse?

Answer 22

The Moon has just finished covering the Sun

Question 23

What is the Fourth Umbral Contact for a Solar Eclipse?

Answer 23

The Moon is just about to exits the Sun’s Umbra

Question 24

What is an Annular eclipse?

Answer 24

An annular eclipse is when the Moon’s silhouette is slightly smaller than an normal eclipse and a ‘ring of fire’ occurs as the Moon is in its apogee in its elliptical orbit around Earth

Question 25

When do Lunar Eclipses occur?

Answer 25

Lunar Eclipses occur when the Moon passes through the Earth's shadow.

Question 26

What happens during a Lunar Eclipse?

Answer 26

During a Lunar Eclipse at totality, the Moon appears red in colour due to predominantly red light refracting through the Earth's atmosphere and illuminating the Moon

Question 27

What is the First Umbral Contact for a Lunar Eclipse?

Answer 27

When the Moon first enters the Earth's Penumbra

Question 28

What is the Second Umbral Contact for a Lunar Eclipse?

Answer 28

The second Umbral contact is when the Moon is completely in the Umbra of the Earth

Question 29

What is the Third Umbral Contact for a Lunar Eclipse?

Answer 29

When the Moon first exits the Umbra

Question 30

What is the Fourth Umbral Contact for a Lunar Eclipse?

Answer 30

When the Moon completely exits the Penumbra

Question 31

Who calculated the average circumference of the Moon?

Answer 31

Eratosthenes

Question 32

How did Eratosthenes calculate the circumference of the Earth?

Answer 32

He read that at noon on the Summer Solstice, columns of temples did not cast any shadows in Syene (Tropic of Cancer). At the same time in Alexandria, the Sun’s position was about 7° (roughly one 50th of a circle) from the zenith. He knew that the distance from Syene to Alexandria was 790 km and used simple Geometry to work out that the circumference of the Earth was 50 x this. He worked this out to be 39,500 km. His reading was very accurate as it was in 5% of the actual circumference of the Earth

Question 33

What assumptions did Aristarchus to calculate the Moon's diameter?

Answer 33

He determined: - A lunar eclipse was produced when the Moon passed into the Earth's Umbra - The Sun was so far away that its rays were parallel when reaching the Earth - The Moon's path took it through the centre of the Earth's Umbra which has the same diameter of the Earth

Question 34

How did Aristarchus calculate the ratio of the Moon's diameter: the Earth's diameter?

Answer 34

Using his three assumptions, Aristarchus showed that: diameter of Moon . .Time interval from U1 to U2 -------------------------- = --------------------------------------- diameter of Earth . . Time interval from U2 to U4 He calculated the Moon's diameter was between 0.32 and 0.40 times of the Earth's. The correct value is 0.27

Question 35

What did Archistarchus measure to deduce the distance to the moon?

Answer 35

The width of his thumbnail

Question 36

What did Eratosthenes use to measure the circumference of the Earth?

Answer 36

Lengths of Shadows at local noon

Question 37

What did Eratosthenes use to measure the distance to the Sun?

Answer 37

The angle between the Sun and Moon at half-moon

Question 38

What did Archistarchus use to measure the diameter of the moon?

Answer 38

Timings of Umbral Contacts during total lunar eclipse

Question 39

What did Eratosthenes do to calculate the diameter of the Sun?

Answer 39

He compared the apparent angular sizes of the Moon and Sun