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Earth orbit
Earth orbit
The path the Earth takes around the Sun.
The Earth spins on an axis, which is an imaginery line drawn between the North Pole and the South Pole. On average the axis tilts 23.5° (degrees) to the plane in which the Earth orbits the Sun, but the angle at which the Earth is tilted relative to the Sun varies. At present it is 21.5°, but over
a period of 40,000 years it can become over 3° larger than this. When the Earth tilts more, there are bigger differences between seasons; when the Earth tilts
less, the differences between the seasons are smaller.
The Earth also changes its orbit around the Sun in a regular way. At times it is almost circular, but over a period of 97,000 years it changes shape until it follows a path like
a giant oval (an ellipse). At those times the Earth is cooler in both summer and winter.
Earth
These ideas about the Earth’s changing movements were used by the Serbian scientist Milutin Milankovitch (1879-1958) to suggest underlying causes for changes in the Earth’s climate and for the start of the Ice Ages.
The Earth’s magnetic north and south poles are not in the same places as the geographic poles.
At present the magnetic poles
are at about 76° north, 100° west (in Canada) and 66° south, 139° east (the Antarctic coast facing Australia).
Eccentric
Another word for elliptical, as in an elliptical (oval) orbit.
Eclipse
An eclipse occurs when the normal sunlight received by the Earth or the Moon is blocked because the paths of the Moon and Earth cross.
Sometimes the Earth blocks out light reaching the Moon, making an eclipse of the Moon; more spectacularly, the Moon blocks out light reaching the Earth, making an eclipse of the Sun.
During an eclipse only a small band of the Earth (at most 270km across) experiences total darkness (this area is called an umbra). But for people in this small band the sky suddenly darkens, and stars become visible.
A much larger band of the Earth’s surface experiences partial darkness (an area called the penumbra); here people still see a milky white sky.
The Sun’s flaring surface, or corona, becomes visible only during an eclipse, which is
why this event is so important
to scientists studying the Sun. (See also: Conjunction; Diamond ring; Lunar eclipse.)
Moon
Umbra Penumbra
 Eclipse—During a solar eclipse the Moon prevents any light from reaching a small area
of the Earth’s surface called the umbra. Beyond this spot a large part of the Sun is obscured, and people see a partial eclipse. This larger area is called a penumbra.
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