Some fun and startling facts about the solar system
Mostly about the earth and moon and Uranus for now.
We know that the earth rotates around its axis approximately once in 24 hours. Assuming the earth is a perfect sphere, what is the speed of a point on the equator?
Answer: The radius of the earth at the equator is r = 6341 km = 3,963 miles. The circumference of the earth at the equator is therefore 2x pi x r = 39,840 km = 24,900 miles. A point on the equator traverses this distance in 24 hours so the speed of the point is ~ 1038 miles/hour or 1,660 km/hour. This is not so fast compared to how fast jet aircraft can fly. The fastest jet plane on record at the moment is the NASA X-43, an experimental aircraft that reached a speed of Mach 9.6, or approximately 7,366 mph (11,854 km/h).
How fast does the earth travel in its orbit around the sun?
Answer: The distance of the earth from the sun is 93.5 million miles and it travels around the sun in 365 days. Hence, in one day, it will travel 2 x pi x 93.5/365 million miles/day. Converting this into miles per second gives the speed of the earth in its orbit as18.6 miles/second. This means that in 1 hour, the earth moves about 67,000 miles forward in its orbit.
How fast does the moon travel in its orbit around the earth?
Answer: The moon is at a distance of approximately 238,900 miles from the earth and its orbital period is 27 days. This gives its orbital speed as 0.64 miles/second. Each day the moon travels 1/27 th of its motion around the earth. This translates into a delay of approximately53 minutes each day. The moon reaches the same point in the sky after 24 hours and 53 minutes each day.
If you stood on the surface of the moon and looked up at the earth, want would be the most astonishing thing you would notice about it?
Answer: The moon keeps the same face turned towards the earth. The most astonishing thing you would notice is that the earth does not move at all, it just hangs in one spot in the sky. You would see it rotate on its axis once in 24 hours, but it would not move from this one spot. In fact, the earth is not visible at all from the side of the moon facing away from the earth (the so called “dark side”). The Soviet spacecraft Luna 3 took the first images of this side in 1959. A detail map of the far side was finally made in 1967 by a US uncrewed mission called Lunar Orbiter 5.
Does the earth rotate East to West or West to East?
Answer: West to East. Since the position of the sun is more or less fixed in one day, this make the sun seems to move East to West in the earth’s sky.
The axis of rotation of the earth tilts about 23 degrees from the plane of its orbit around the sun. This is the reason why polar regions on the earth have long nights in winter and long days in the summer. The planet Uranus has the most unusual axis of rotation tilt: the axis is almost in the plane of its orbit (~ 98 degrees tilt). How would the sun seem to move in the sky on Uranus (approximately) if you stood at the north pole of this planet?
Answer: Uranus rotates on its axis in ~ 17.25 hours and revolves around the sun in ~ 84 earth years. Since the axis of rotation of the planets always points in the same direction relative to the fixed stars (because of angular momentum conservation), the sun would be visible from its northern hemisphere for 42 years and invisible for 42 years. At the height of summer at the north pole, the sun would be high in the sky and then make slowly widening concentric circles in the sky each 17.25 hours, moving from the north to the south. These circles would reach the horizon in 21 years. After that, for 42 years, the sun would be invisible from the northern hemisphere and the concentric circular motion of the sun would be seen in the southern hemisphere, moving from the horizon up to the zenith for 21 years and then back down again for another 21 years. After that the sun would emerge again in the northern hemisphere at the horizon and start its journey up to the zenith, reaching it in 21 years.
6. a. What is the length of a day on the moon? b. And does the moon have earth and/or solar eclipses?
Answer: The moon’s rotation axis is about 6 degrees from its orbital axis so it rotates almost perpendicular to its orbital plane. The time it takes for the moon to complete an orbit around the earth is 27.32 earth days – this is the time it takes to come back to the position it had. However, the earth/moon system has moved forward around the sun so as seen from the earth, the time between two full moons is a bit longer – about 29.53 earth days.
a. Starting with the sun at the zenith at the lunar equator, as the moon moves around the earth, it will experience sunset after 7.38 earth days. Then there will be a period of darkness for 14.76 earth days after which the sun will rise again at the opposite spot from where it set and then take 7.38 earth days to reach the zenith again. Thus the moon has about two weeks (earth time) of daylight followed by about 2 weeks of darkness. For more northern or southern latitudes, the sun is lower down in the sky just like on the earth. At the lunar rotational poles, the sun makes a circle around the horizon tilted by ~ 6 degrees.
b. The moon does have solar eclipses, which happen when the earth overshadows the sun. But these are restricted to the side of the moon facing the earth. But there are no earth eclipses as those would require the sun to block out the earth, which cannot happen.