Back to: BASIC SCIENCE TECHNOLOGY JSS 1
Welcome to class!
In today’s class, we will be talking more about the earth in space. Enjoy the class!
Earth in Space II
Earth rotation and revolution
The term earth revolution is used to describe the path (or orbit) of Earth through space. Earth’s revolution around the sun is responsible for seasonal change and leap years. This path is shaped like an ellipse and has points when Earth is closer to the sun and farther from it.
Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, and 4 seconds with respect to other, distant, stars (see below). Earth’s rotation is slowing slightly with time; thus, a day was shorter in the past.
Eclipse of the sun
The apparent or angular sizes of both the Sun and Moon vary slightly from time to time as their distances from Earth vary. Much of the time, the Moon looks slightly smaller than the Sun and cannot cover it completely, even if the two are perfectly aligned. In this type of “annular eclipse,” there is a ring of light around the dark sphere of the Moon.
However, if an eclipse of the Sun occurs when the Moon is somewhat nearer than its average distance, the Moon can completely hide the Sun, producing a total solar eclipse. Another way to say it is that a total eclipse of the Sun occurs at those times when the umbra of the Moon’s shadow reaches the surface of Earth.
Eclipse of the moon
A lunar eclipse occurs when the Moon enters the shadow of Earth. Earth’s dark shadow is about 1.4 million kilometres long, so at the Moon’s distance (an average of 384,000 kilometres), it could cover about four full moons.
Unlike a solar eclipse, which is visible only in certain local areas on Earth, a lunar eclipse is visible to everyone who can see the Moon. Because a lunar eclipse can be seen (weather permitting) from the entire night side of Earth, lunar eclipses are observed far more frequently from a given place on Earth than are solar eclipses.
An eclipse of the Moon is total only if the Moon’s path carries it through Earth’s umbra. If the Moon does not enter the umbra completely, we have a partial eclipse of the Moon. But because Earth is larger than the Moon, its umbra is larger, so that lunar eclipses last longer than solar eclipses, as we will discuss below.
A lunar eclipse can take place only when the Sun, Earth, and Moon are in a line. The Moon is opposite the Sun, which means the Moon will be in the full phase before the eclipse, making the darkening even more dramatic. About 20 minutes before the Moon reaches the dark shadow, it dims somewhat as Earth partly blocks the sunlight. As the Moon begins to dip into the shadow, the curved shape of Earth’s shadow upon it soon becomes apparent.
In our next class, we will be talking about Satellites. We hope you enjoyed the class.
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