Encyclopedic Entry

This solar eclipse gives viewers a perfect "diamond ring."

Photograph by Vidur Parkash, MyShot

Turn on a Light, Would You?
A solar eclipse in July 2009, visible mostly to people in China, India, Nepal, and parts of the Philippines, was the longest solar eclipse so far in the 21st century. It lasted a whopping six minutes and 39 seconds!

Mark Your Calendar
Transits of Mercury and Venus happen very rarely. There are only 13 transits of Mercury every century, and even fewer transits of Venus. The next transit of Venus will happen on June 6, 2012, at 1:28 p.m. (The next transit of Venus will happen in 2117!)

The NASA Eclipse website provides the days and times of transits of Mercury and Venus through the year 2368.

Theres a lot of movement going on in our solar system. All the planets constantly move at different speeds around the sun. Moons constantly spin around each planet. Every now and then, one of these objects moves into the shadow of another one. We call this an eclipse.

Solar Eclipse

A solar eclipse happens when the Moon, on its journey around the Earth, happens to pass between the sun and the Earth for a moment. A solar eclipse can only happen during a new moon, a phase of the lunar cycle where the Moon lies between the Earth and the sun. At night, the Moon is totally dark during a new moon.

If everything lines up just right, the sun is fully or partially covered by the Moon. That means those of us on Earth see a big, round shadow (the Moon) sliding over and covering the sunlight we normally see in the daytime sky. The shadow covering the sun is called an umbra.

Total solar eclipses, where the moon perfectly blocks out the entire sun, are rare. Partial solar eclipses, where the moon covers only a part of the sun, are much more common.

Between two and five total or partial solar eclipses happen every year. Whether or not you see a full or partial eclipse depends on where you are on Earth. A full eclipse may be visible to people in the Northern Hemisphere when it is facing the sun, for example. In this situation, people in the Southern Hemisphere will not see the eclipse at all.

Solar eclipses are usually over and done with pretty quickly. Scientists say total solar eclipses, in the very best conditions, can only be seen for about 7 minutes. The Moons umbra keeps sliding over until the sun peeks through again.

The Moon is just the right size to perfectly cover the sun. The sun is actually many, many, many, times larger than the Moon, but the Moon is much closer to the Earth, so it appears larger.


Lunar Eclipses

Lunar eclipses happen when the Moon passes through the Earths shadow. This can only happen when the Moon is on the far side of the Earth, away from the sun. Think of the Earth in the center, with the sun on one side, and the moon on the other. So, lunar eclipses only happen when there is a full moon.

Lunar eclipses are much more common than solar eclipses, and many more people can see them at the same time. In fact, a whole hemisphere can usually see a lunar eclipse happening. Part of why theyre easier to see is that they last longer; lunar eclipses last several hours.

Other Eclipses

As other planets pass in front of the sun, they can also cause eclipses. These eclipses are called transits. From Earth, we can only see transits of Mercury and Venus, the two planets closer to the sun than Earth. From the planet Saturn, though, a satellite may view transits of Earth, Mercury, and Jupiter as well.

To view the transit of Mercury or Venus requires a telescope. The sun is so massive that even with powerful telescopes, the planets look like grains of sand moving across a giant beach ball.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

constantly

Adverb

always.

Earth

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Encyclopedic Entry: Earth

eclipse

Noun

an event where one heavenly body obscures another.

Encyclopedic Entry: eclipse

hemisphere

Noun

half of a sphere, or ball-shaped object.

Encyclopedic Entry: hemisphere

interesting

Adjective

exciting or able to hold attention.

lunar cycle

Noun

complete cycle of visible phases of the moon, approximately 29.5 days.

lunar eclipse

Noun

event wherein the moon passes through the Earth's shadow.

Mercury

Noun

smallest planet in the solar system, and closest to the sun.

Moon

Noun

Earth's only natural satellite.

new moon

Noun

Dark phase of the lunar cycle when the moon is invisible or barely visible, occurring when the moon passes between the sun and earth.

Northern Hemisphere

Noun

half of the Earth between the North Pole and the Equator.

partial solar eclipse

Noun

event in which the sun is partly blocked by the moon passing between it and the Earth.

peek

Verb

to look quickly or from a secret location.

planet

Noun

large, spherical celestial body that regularly rotates around a star.

Encyclopedic Entry: planet

rarely

Adverb

not often.

satellite

Noun

object that orbits around something else. Satellites can be natural, like moons, or made by people.

Saturn

Noun

sixth planet from the sun.

shadow

Noun

dark area where an object prevents light from reaching a surface.

solar eclipse

Noun

event when the sun is blocked by the moon passing between it and the Earth.

solar system

Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

Southern Hemisphere

Noun

half of the Earth between the South Pole and the Equator.

sun

Noun

star at the center of our solar system.

telescope

Noun

scientific instrument that uses mirrors to view distant objects.

total solar eclipse

Noun

event when the sun is entirely blocked by the moon passing between it and the Earth.

transit

Noun

event when a planet passes between a viewer and the sun.

umbra

Noun

moon's shadow that covers the sun during a solar eclipse.

Venus

Noun

planet in the solar system, second from the sun.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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