An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer. An eclipse is the result of either an occultation (completely hidden) or a transit (partially hidden).
A solar eclipse is when the earth's moon passes between the sun and the earth to block the view of the sun on the earth. A lunar eclipse is when the moon passes through the shadow of the earth when the earth comes between the sun and the moon. A total eclipse completely blocks the view while a partial eclipse justs covers a portion of the viewed object.
For any two objects in space, a series of lines can be drawn to explain what causes an eclipse and to predict how it will been seen at a third point in space. Typically the cross-section of the objects involved in an astronomical eclipse are roughly shaped like a circle. The region of an object's shadow during an eclipse is divided into three parts:
- The umbra, within which the object completely covers the light source. For example, the moon's shadow where the sun's light is completely blocked is in the shape of a cone.
- The antumbra, extending beyond the tip of the umbra, within which the object is completely in front of the light source but too small to completely cover it. Light travelling from the edges of the light source radiate beyond the shadow.
- The penumbra, within which the object is only partially in front of the light source.
A total eclipse occurs when the observer is within the umbra, an annular eclipse when the observer is within the antumbra, and a partial eclipse when the observer is within the penumbra. (See diagram at left).
For spherical bodies, when the occulting object is smaller than the star, the length (L) of the umbra's cone-shaped shadow is given by:
where Rs is the radius of the star, Ro is the occulting object's radius, and r is the distance from the star to the occulting object.
Because the paths of the objects in the solar system are well understood, astronomers can predict with great accuracy when and where eclipses will be observed.
Because observing an occultation from a stationary position on earth may only last for a couple of minutes, scientists use jet airplanes to move instruments rapidly to compensate for the rotation of the earth. These measurements can lead to important discoveries. For example, astronomers used a jet to observe the occultation of the star SAO 158687 (also known as HD 128598) by Uranus to study its atmosphere, but unexpectly the data showed that Uranus had planetary rings.
Biblical references to an eclipse
There are nine primary biblical references to an eclipse:
- Isaiah 13:10 ("For the stars of heaven and their constellations Will not flash forth their light; The sun will be dark when it rises And the moon will not shed its light.")
- Ezekiel 32:7 ("And when I extinguish you, I will cover the heavens and darken their stars; I will cover the sun with a cloud And the moon will not give its light.")
- Joel 2:10 ("Before them the earth quakes, The heavens tremble, The sun and the moon grow dark And the stars lose their brightness.")
- Joel 2:31 ("The sun will be turned into darkness And the moon into blood Before the great and awesome day of the LORD comes.")
- Joel 3:15 ("The sun and moon grow dark And the stars lose their brightness.")
- Matthew 24:29 ("But immediately after the tribulation of those days THE SUN WILL BE DARKENED, AND THE MOON WILL NOT GIVE ITS LIGHT, AND THE STARS WILL FALL from the sky, and the powers of the heavens will be shaken.")
- Mark 13:24 ("But in those days, after that tribulation, THE SUN WILL BE DARKENED AND THE MOON WILL NOT GIVE ITS LIGHT.")
- Revelation 6:12 ("I looked when He broke the sixth seal, and there was a great earthquake; and the sun became black as sackcloth made of hair, and the whole moon became like blood;")
- Revelation 8:12 ("The fourth angel sounded, and a third of the sun and a third of the moon and a third of the stars were struck, so that a third of them would be darkened and the day would not shine for a third of it, and the night in the same way.")
- Elliot, J. L.; Dunham, E.; Mink, D. (1977). "The rings of Uranus". Cornell University 267: 328–330. doi:10.1038/267328a0. http://www.nature.com/nature/journal/v267/n5609/abs/267328a0.html. Retrieved June 9, 2007.