Eclipses, including total solar eclipses, have been the subject of superstition and scientific curiosity throughout history. There was a time when some cultures dreaded eclipses, but many people look forward to their occurrence in modern times. timeanddate.com explores how a total solar eclipse works and briefly examines other types of eclipses, including lunar eclipses.
The above image shows an example of a general total solar eclipse. ©iStockphoto.com/Simon Podgorsek
How Does a Total Solar Eclipse Work?
A total solar eclipse occurs when the moon completely covers the sun, as seen from earth. Total solar eclipses happen because the sun is near one of the nodes of the lunar orbit, and the moon is at perigee (the point in the orbit of the moon or a satellite at which it is nearest to the earth) at this node at the same time.
During totality the photosphere, a blindingly bright surface of the sun that is visible on a sunny day, is hidden and allows other normally invisible features of the sun to be seen. Surrounding the photosphere is a thin, deep red layer of the sun called the chromosphere. Measuring a few thousand kilometers thick, the chromosphere is seen for only a few seconds at the beginning and end of totality. Flame-like prominences from behind the moon’s silhouette stretch for thousands of kilometers into space. The white corona encircles the eclipsed sun and extends the sun’s diameter several times.
Timing and location are important to view a total solar eclipse. By the time it reaches earth, the moon’s umbra is about 270 kilometers (170 miles) across at its widest so a chance that the umbral shadow passes over at a particular spot is slim. A total solar eclipse at one given point on earth occurs on average only once in about 360 years.
The sun’s diameter is about 1,392,000 kilometers (844,900 miles) and the moon’s diameter is about 3476 kilometers (2160 miles) in comparison. This works out to be a ratio of approximately 400 to one – the sun is about 400 times larger in diameter than the moon. At the same time, the sun is about 149,600,000 kilometers (93,000,000 miles) from earth while the moon is about 384,500 kilometers (240,000 miles) away. Both distances vary over time.
As a result, the moon and sun each appear the same size in the sky – about half a degree. When a solar eclipse occurs, the moon appears to cover the sun’s surface while exposing the sun’s normally invisible chromospheres. An eclipse’s duration is longer if the moon-to-sun ratio is greater.
Interesting Fact
Totality of a solar eclipse can last about seven minutes and 31 seconds but these eclipses are rare. It is predicted that totality during the solar eclipse will last for seven minutes and 14 seconds in the year 2150, which is longer than any total solar eclipse since the ninth century CE. A period of totality is normally shorter than five minutes.
Other Solar Eclipses
While a solar eclipse is seen over only a small part of the day side of earth and changes in appearance, a lunar eclipse can be viewed from across the entire night side of the earth (depending on weather) and will appear exactly the same for all observers on the earth’s night hemisphere. A solar eclipse falls in the following categories:
- The total solar eclipse.
- The partial eclipse.
- The annular eclipse.
A partial solar eclipse occurs when only the lunar penumbra (the partially shaded outer region of the shadow cast by an object) touches the earth and the umbra casts off into space and misses the earth. The percentage of the sun varies depending on the observer’s location but regardless of a viewer’s position, the sun will only be partially eclipsed. Annular eclipses occur when the moon appears smaller than the sun as it passes centrally across the solar disk and a bright ring, or annulus, of sunlight remains visible during the eclipse.
There is also a “hybrid” eclipse, which is a type of central eclipse where parts of the path are annular while other parts are total. At the extremes of the eclipse’s solar central path, the moon appears too small to mask the sun entirely and yields an annual eclipse. However, due to earth’s curvature, the moon’s size appears to increase to block the entire solar disk and produces a very short total eclipse. These eclipses are rare. The next solar eclipse is an annular eclipse, which will occur on January 26, 2009. The next total solar eclipse occurs on July 22, 2009.
Solar Eclipse in Society
The solar eclipse has been linked to many ancient customs and traditions over history. These days many eclipse enthusiasts across the world wait eagerly for a solar eclipse to occur so they can experience and remember the moment. timeanddate.com also offers some basic tips for solar eclipse enthusiasts.
Lunar Eclipses in a Nutshell
The lunar eclipse occurs when the moon passes through the earth’s shadow and can only happen at a full moon. One of the major differences between a lunar and solar eclipse is that a lunar eclipse can be viewed from across the entire night side of the earth (depending on weather). Types of lunar eclipses include:
- The total lunar eclipse, which occurs when the moon’s travels completely into the earth’s umbra. The moon never complete disappears during a total lunar eclipse.
- The partial lunar eclipse, which occurs when the moon is oriented in a way that only part of it dips into the earth’s umbra.
- The penumbral lunar eclipse, which occurs when the moon passes through the faint penumbral portion of the earth’s shadow.
There is always a full moon on the night of a lunar eclipse. The eclipse’s type and length depends on the moon’s location relating to its orbital nodes (one or two points where an orbit crosses a plane of reference that it is inclined to). A lunar eclipse has two magnitude values – the penumbral magnitude and the umbral magnitude.
A popular activity during a partial lunar eclipse viewing is to videotape the eclipse through a telescope. However, others may prefer a lightweight camcorder for later analysis.
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