Chapter 4

Q&A from Chapter 4 – The Earth As A Planet. If there is anything wrong, leave a comment and I will adjust whatever I said wrong.

1a) Give two reasons why the Earth is different from all the other planets in the Solar  System.
It is the only planet in the solar system that is suitable for advanced life such as our own. It is also the right distance from the sun so that it has a certain temperature suitable to stabilise life, and suitable to sustain liquid water.

1b) What is the true shape of the Earth?
It is an oblate spheroid, which is a sphere that bulges at the waist or in our case, the equator. It is this shape due to the rotating axis.

1c) Explain what is meant by the terms perihelion and aphelion.
Because the Earth has an elliptical orbit, the distance between the Earth and Sun vary. At it’s closest point, perihelion, it is roughly 147,000,000 km. The aphelion is the furthest away point from the sun, which is approximately 152,000,000 km.

2a) Why does a star twinkle more when it is low down than when it is high up?
A star twinkles more when low down because there is more atmosphere between the observer and the star. An atmosphere diffuses the star’s light slightly, so when it is seen in the sky, it will have ‘rays’ coming from a central point, which is the light being diffused. So if a star is low down, it is diffused more than a star higher up; which has less atmosphere to travel through.

2b) Why does the Sun often look very red when setting?
When the Sun is low down in the sky, again, because of the atmosphere, the blue light from the Sun cannot get through as easily as the red light can, so we see the red sunset.

2c) Why is the sky blue?
Because of diffusion within the atmosphere, the blue light from the Sun (during the day) is scattered around in the atmosphere, making the sky look blue.

3a) Explain what is meant by latitude and longitude. What is (i) the latitude of the equator, (ii) the latitude of the south pole, (iii) the longitude of Greenwich Observatory?
Latitude:    A point somewhere on the Earth’s surface with an angular distance from    the equator.
Longitude:    A point somewhere on the Earth’s surface with an angular distance from the Greenwich Meridian, which is taken as 0°.
(i) The latitude of the equator is 0° because that is the starting point of measuring latitudes.
(ii) The latitude of the south pole, would be 90°, as it is a right angle from the equator.
(iii) The longitude of Greenwich Observatory is 0°, because like the latitude of the equator, it is a staring point of measuring longitudes.

3b) Using diagrams, give an explanation of the phenomenon of the seasons.
Because the Earth is inclined by 23.5°, then at different times in the year, different parts of the Earth are more inclined towards to Sun than others are. This means they will generally have a longer period of light and warmer temperatures. In northern summer, the North Pole is inclined 23.5° towards the Sun, which heats the northern hemisphere. In northern winter, the North Pole is inclined 23.5° away from the sun, which cools the northern hemisphere. These are the causes of seasons.Diagram representing the Seasons Cycle

3c) What is meant by the term equinox?
The Sun does not move along the celestial equator, this means that in a simple observation, the sun appears to go up and down across the equator. Equinox is the point, where the sun crosses the equator, resulting in exact same day and night lengths on both hemispheres.

4a) During June, two boys took their holidays at the same time, one in Norway and the other in Greece. When they compared notes, they found that the boy who had been to Norway had had more hours of daylight than his friend in Greece. Why?
During northern summer, the North Pole is in constant daylight, which means as closer to the equator you go from the north pole, the less daylight you will have. Because the boy in Norway was closer to the north pole than the boy in Greece, he had more daylight.

4b) Is it correct to say simply that the Sun rises in the east and sets in the west?
NO! Firstly, the Sun isn’t moving; it is the Earth that moves. Secondly, the Earth therefore does not move east to west because then the Sun would seem to move west to east. This means the Earth must move west to east.

4c) Why does not every place on Earth have 12 hours of daylight followed by 12 hours of darkness?

To achieve that, the Sun would have to move along the celestial equator and stay perfectly in line with the equator. However, the Sun does not do that but varies by going above and below the equator. Therefore, constant 12 hours shifts of light and darkness are not possible.
5a) Why does Polaris seem to remain almost motionless in the sky, with all other celestial bodies apparently moving round it?
The north celestial point is represented by the bright star, however, the star is not exactly the north celestial point. This means that as the Earth rotates on it’s axis, a singularly stood observer would see Polaris move in a very small circular movement because the Earth moved in a circular movement. Same with all the other stars, as they move, they are moving in a circular motion because of the Earth’s movement. N.B The Stars Never Move, It Is Always The Earth.

Long Exposure Photo show the stars movement

5b) What view of Polaris would you have from (i) the north pole of the Earth, (ii) London (latitude 51.5°N), (iii) the equator, (iv) New Zealand?
The view of Polaris from the north pole would be directly above you, but possibly a chance of a very small circular motion.
The view of Polaris from London would be roughly 40° from the horizon.
The view of Polaris from the equator would be on the horizon, with the possibility of it disappearing once in a while due to the Earth’s rotation.
The view of Polaris from New Zealand would be none existent. Polaris can only be seen in the northern hemisphere, and New Zealand is in the southern hemisphere. It simply cannot be seen.

5c) Explain the phenomenon of precession.
The bulge at the equator of Earth can cause a very slow phenomenon called precession. The Sun and Moon pull on this bulge, and cause the Earth’s axis to wobble slightly. It takes about 25, 800 years to fully complete a cycle of precession. For instance, when the Egyptians were building the pyramids, the polar point did not lay near Polaris, but another smaller, dimmer star. In 12,000 years from now, the polar point will be closer to larger, bright, blueish star called Vega. But 25,800 years from now, the polar point will again be near Polaris. It is a cycle.

6a) What is meant by saying that a star is circumpolar?
Circumpolar refers to the motion of a star near Polaris that manages to complete a while circle without going below the horizon. Simply, if the star was tracked for a whole day, then it would draw a circle in the sky.

6b) What exactly is the Zodiac?
The Zodiac is a collection of constellations, which the Sun passes through as the Earth orbits the sun. For instance, in late October, the Sun is in the constellation Scorpius (The Scorpion).

6c) How would the apparent movements of the bodies in the sky be affected if the Earth rotated on its axis once in 20 hours, and took 400 days to complete one journey round the Sun?
If the Earth rotated on its axis once every 20 hours then the speed of the bodies in the sky would increase. However the rate that the bodies in the sky move (i.e. Like the Zodiac) would decrease, so the same stars would be seen every night for a very long time.


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