Perhaps the most spectacular astronomical events that one can observe without a telescope, lunar and solar eclipses were considered omens of great fortune or complete disaster in ancient times. We now know that the occurrence of eclipses is a consequence of the orbits of the Earth and Moon with respect to the Sun.
The Earth orbits the Sun once a year, and the Moon orbits the Earth once a month; it turns out that the planes of the Moon's and Earth's orbits are almost, but not quite, aligned (the offset is about 5 degrees). This means that every once in awhile (a few times a year), the Moon passes through the Earth's shadow at night, blocking our view of the Moon: we call this a lunar eclipse. Similarly, the Moon can come between the Sun and the Earth during the day, temporarily blocking the Sun from view: these events are called solar eclipses. Thus contrary to the ancients' beliefs, these alignments are not conjured up by the divine but are predictable consequences of the Moon's and Earth's orbits in the solar system. It is quite fortuitous, however, that despite their vastly different physical extents, the Sun and the Moon appear to be about the same size in the sky (the Sun is much, much bigger than the Moon, but also much farther away from the Earth). So, when a solar or lunar eclipse occurs, it is possible for one object to completely obscure the other from view: this is called a total eclipse. It is more common, however, that only part of the Moon or Sun is obscured, in which case we call it a partial eclipse.
Though a total solar eclipse may be seen more than once a year on Earth, from a given spot on the planet these events are almost as rare as they are spectacular. The relative motions of the Earth and the Moon cause solar eclipses to be visible only within a strip of a few degrees in latitude, and total obscuration lasts no more than about seven minutes. Thus, at any single location on Earth, a total solar eclipse occurs only once every 300 years or so. Because the shadow cast by the Earth is quite a bit larger than the Moon, lunar eclipses are more common than solar eclipses, and totality can last for about an hour. Nonetheless, the beauty of such events entices both professional and amateur astronomers alike to chase them all around the globe!
- Why do we not have eclipses every month? (Beginner)
- What should I know about the upcoming Solar Eclipse (2017)? (Beginner)
- Can a lunar and a solar eclipse happen in the same month? When will this happen next? (Intermediate)
- Why do some eclipses take longer than others? (Intermediate)
- Are there eclipses on other planets? (Intermediate)
- What is the best time of the year to see eclipses? (Intermediate)
- Does Mt. Everest cast a shadow on the moon? (Intermediate)
- !What calculations go into predicting eclipses? (Advanced)
- Why can we have solar eclipses? (Beginner)
- How long does a solar eclipse last? (Beginner)
- Will I be able to see the shadow of the Moon streaking towards me in a total solar eclipse? (Beginner)
- Is there a simple formula to calculate solar eclipse timings from any position on Earth? (Intermediate)
- Is it a coincidence that we can have total solar eclipses? Are there other planets which also have them? (Intermediate)
- Will we ever stop having solar eclipses because of the moon's motion away from the Earth? (Intermediate)
The Ask an Astronomer team's favorite links about Lunar and Solar Eclipses:
- NASA Eclipse Web Site (Espenak): Great site with detailed information on five thousand years of solar and lunar eclipses
- MrEclipse.com: Help on eclipse photography
- solarpower.guide: Guide to Eclipses
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