Eclipses

 

 

Because of the tilt of the Moon’s orbit, the Moon usually passes above or below the Sun at new moon and above or below Earth’s shadow at full moon. Sometimes, though, the full moon or new moon crosses the plane of Earth’s orbit. By a coincidence of nature, even though the Moon is about 400 times smaller than the Sun, it is also about 400 times closer to Earth than the Sun is, so the Moon and Sun look almost exactly the same size from Earth. If the Moon lines up with the Sun and Earth at new moon (when the Moon is between Earth and the Sun), it blocks the Sun’s light from Earth, creating a solar eclipse. If the Moon lines up with Earth and the Sun at the full moon (when Earth is between the Moon and the Sun), Earth’s shadow covers the Moon, making a lunar eclipse.

A total solar eclipse is visible from only a small region of Earth. During a solar eclipse, the complete shadow of the Moon that falls on Earth is only about 160 km (about 100 mi) wide. As Earth, the Sun, and the Moon move, however, the Moon’s shadow sweeps out a path up to 16,000 km (10,000 mi) long. The total eclipse can only be seen from within this path. A total solar eclipse occurs about every 18 months. Off to the sides of the path of a total eclipse, a partial eclipse, in which the Sun is only partly covered, is visible. Partial eclipses are much less dramatic than total eclipses. The Moon’s orbit around Earth is slightly elliptical, or egg-shaped. The distance between Earth and the Moon varies slightly as the Moon orbits Earth. When the Moon is farther from Earth than usual, it appears smaller and may not cover the entire Sun during an eclipse. A ring, or annulus, of sunlight remains visible, making an annular eclipse. An annular solar eclipse also occurs about every 18 months. Additional partial solar eclipses are also visible from Earth in between.

At a lunar eclipse, the Moon is actually in Earth’s shadow. When the Moon is completely in the shadow, the total lunar eclipse is visible from everywhere on the half of Earth from which the Moon is visible at that time. As a result, more people see total lunar eclipses than see total solar eclipses.

Gravity

Try to jump as high as you can. Bend your knees. Now jump! No matter how hard you try, or how high you jump, you always come back down again.

Something called gravity pulls you back down. Gravity holds you down on Earth. Without gravity, you would fly off into space. You would jump up and just keep on going. That might sound like fun, but you could not live very high up. For one thing, you need to breathe air. The higher you go, the less air there is. You need gravity to keep you down on Earth.

WHAT IS GRAVITY?

Gravity is a force that pulls two objects toward each other. The force is also called gravitation. The bigger an object is, the stronger its gravity. Small objects, and even cars and buildings, have so little gravity that you can’t feel their pull. Huge objects like Earth, however, are a different story.
Earth is so much bigger than you that its gravity makes you “stick” to the ground. Earth is bigger than the Moon. Earth’s gravity pulls on the Moon. That is why the Moon orbits, or goes around, Earth. The Sun is bigger than Earth. Its gravity makes Earth go around the Sun.

Earth’s gravity pulls toward the center of Earth. Gravity holds the oceans and the atmosphere on Earth. The Sun’s gravity pulls toward the center of the Sun. It holds the Sun together.

WHO DISCOVERED GRAVITY?

Gravity has always been around. In ancient times, people tried to explain why things fall toward Earth. An English scientist named Sir Isaac Newton came up with a great idea about gravity in 1687. He thought about how an apple falls and wondered how far gravity went. He came up with the idea that gravity does more than hold people on Earth.

Newton thought of gravity as a kind of mysterious force pulling objects together. He said that gravity holds the Moon in orbit around Earth. It holds the planets in orbit around the Sun. Newton’s ideas on gravitation explained many things about how apples fall and how stars and planets move.

A DENT IN SPACE-TIME

In the early 1900s, another scientist named Albert Einstein came up with a very different idea of what gravity is. Einstein had some pretty amazing ideas. He said there was something called space-time. He thought of gravity as a dent in space-time.

This is not as hard to imagine as it might seem. Think about placing a bowling ball on a soft mattress. The mattress is like space-time. The bowling ball is like a star. The bowling ball makes a dent in the mattress. This is sort of how a star dents space-time. If you try to roll a tennis ball past the bowling ball, it curves around the bowling ball as it passes through the dent.

The planets in our solar system roll around the dent in space-time made by the Sun. The dent keeps planets from going off into space. The dent in space-time keeps the planets going around the Sun.

Scientists are still not sure what causes gravity. Finding out more about gravity is an important problem for the future.