Topic 3 - The Earth-Moon-Sun systems

Question 1

What is the distance between the Earth and the Moon?

Answer 1

385, 000km

Question 2

What is the distance between the Earth and the Sun?

Answer 2

150 million km

Question 3

How did Aristarchus calculate the distance to the Moon?

Answer 3

• He noticed that his thumb at arm’s length covered the Moon
• He measured his thumb and arm
• He found the Moon subtended (covered) an angle of 2°
• He used trigonometry and the diameter of the Moon to calculate distance - 100 800km
• Very off as actual angle subtended was 0.5°
• Real answer - 384 400km

Question 4

How did Aristarchus calculate the diameter of the Moon?

Answer 4

• He knew the Moon travelled at a constant speed
• He observed a lunar eclipse
• He used (U1 - U2)/(U2 - U4) and diameter of Earth to calculate diameter of the Moon - 0.32-0.4 times the Earth’s size
• Actual value is 0.27 as he assumed that light rays are parallel and the Moon passes through the centre of the Umbra

Question 5

How did Aristarchus calculate distance to the Sun?

Answer 5

• He found the angle between Sun and Moon at First Quarter to be 87°
• This means that Earth to Sun is 20x further than Earth to Moon
• He was very out as it is hard to accurately tell when Moon is at First Quarter - actual angle is 89° 50’ so Sun was actually 350x further

Question 6

How did Aristarchus calculate diameter of the Sun?

Answer 6

• He noticed that the Moon and Sun both subtend the same angle when viewed with the naked eye from Earth
• As he had already estimated that the Sun was only 20x further from the Earth than the Moon, this would mean that the Sun was only 20x bigger than the Moon
• He was very out as his previous calculations were incorrect

Question 7

What is the mean diameter of the Sun?

Answer 7

1.4 x 10^6 km

Question 8

When can total solar eclipses take place?

Answer 8

During a new moon

Question 9

What is First umbral contact in a Solar eclipse?

Answer 9

The edge of the Moon appears as a small black arc

Question 10

What is Second umbral contact in a Solar eclipse?

Answer 10

Totality begins, corona visible

Question 11

What is Third umbral contact in a Solar eclipse?

Answer 11

Disc of Sun is seen again, corona not visible

Question 12

What is Fourth umbral contact in a Solar eclipse?

Answer 12

Moon’s disc leaves disc of Sun, penumbra leaves observer’s location

Question 13

When can Lunar eclipses occur?

Answer 13

During a full moon

Question 14

What is First umbral contact in a lunar eclipse?

Answer 14

Moon enters Earth’s penumbra, small arc appears to be cut out of Moon’s disc

Question 15

What is Second umbral contact in a lunar eclipse?

Answer 15

Totality begins, Moon appears red

Question 16

What is Third umbral contact in a lunar eclipse?

Answer 16

Totality ends, Moon moves back into Earth’s penumbra, Earth’s shadow leaves Moon’s surface

Question 17

What is Fourth umbral contact in a lunar eclipse?

Answer 17

The Moon moves out of the Earth’s shadow

Question 18

What is an annular solar eclipse?

Answer 18

When the Moon is further from the Earth the Moon does not cover the Sun’s disc completely

Question 19

What is a partial solar eclipse?

Answer 19

The Moon moves across the disc of the Sun but does not block it out completely

Question 20

What is a hybrid solar eclipse?

Answer 20

Annular at some places on the line of totality and total in other places

Question 21

What is the line of totality?

Answer 21

A line along the Earth’s surface where the shadow is focused