3 The Earth-Moon-Sun System Flashcards
How do tidal forces occur?
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
How do tidal forces affect the Earth-Moon system?
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
How many times does the coast experience high or low tides?
There are two daily high and low tides
What are Lunar tides?
Tides caused by the Moon
What are Solar tides?
Tides caused by the Sun
What are Neap tides?
‘Low’ high tides
What are spring tides?
‘High’ high tides
When can spring tides occur?
Spring tides can only occur when the moon is in the full or new phase
Why do spring tides occur?
The positions of the Solar and Lunar tides add up.
When can neap tides occur?
Neap tides can only occur during quarter moons
Why do neap tides occur?
Neap tides occur as the solar tide adds to the lunar low tide, which partially cancels it out
What is another consequence of tidal forces?
Precession
What is precession?
Precession is the change in the orientation of the rotational axis of a rotating body
What is the evidence of precession?
- 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
What is a total solar eclipse?
When the Moon passes directly in front of the Sun , it obscures light for observers and produces a dark shadow.
What is the Umbra?
The area during a Solar eclipse where the Sun is fully covered by the Moon
What is the Penumbra?
The area of observation where the Sun is partially covered
What are Baily’s Beads?
Baily’s beads are when the Sun’s rays shine through the valleys of the Moon and are visible
What is the Diamond Ring effect?
The Diamond Ring effect is when only one Baily bead is visible
What is the First Umbral Contact for a Solar Eclipse?
When the Moon first touches the Sun
What is the Second Umbral Contact for a Solar Eclipse?
The Moon is just about to fully cover the Sun
What is the Third Umbral Contact for a Solar Eclipse?
The Moon has just finished covering the Sun
What is the Fourth Umbral Contact for a Solar Eclipse?
The Moon is just about to exits the Sun’s Umbra
What is an Annular eclipse?
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
When do Lunar Eclipses occur?
Lunar Eclipses occur when the Moon passes through the Earth’s shadow.
What happens during a Lunar Eclipse?
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
What is the First Umbral Contact for a Lunar Eclipse?
When the Moon first enters the Earth’s Penumbra
What is the Second Umbral Contact for a Lunar Eclipse?
The second Umbral contact is when the Moon is completely in the Umbra of the Earth
What is the Third Umbral Contact for a Lunar Eclipse?
When the Moon first exits the Umbra
What is the Fourth Umbral Contact for a Lunar Eclipse?
When the Moon completely exits the Penumbra
Who calculated the average circumference of the Moon?
Eratosthenes
How did Eratosthenes calculate the circumference of the Earth?
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
What assumptions did Aristarchus to calculate the Moon’s diameter?
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
How did Aristarchus calculate the ratio of the Moon’s diameter: the Earth’s diameter?
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
What did Archistarchus measure to deduce the distance to the moon?
The width of his thumbnail
What did Eratosthenes use to measure the circumference of the Earth?
Lengths of Shadows at local noon
What did Eratosthenes use to measure the distance to the Sun?
The angle between the Sun and Moon at half-moon
What did Archistarchus use to measure the diameter of the moon?
Timings of Umbral Contacts during total lunar eclipse
What did Eratosthenes do to calculate the diameter of the Sun?
He compared the apparent angular sizes of the Moon and Sun
