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Fraser
2016-Sep-27, 01:10 PM
http://www.universetoday.com/wp-content/uploads/2016/03/2016-Total-Solar-Eclipse-700x432.jpg
Imagine if you will, that you are a human being living in the prehistoric past. You look up at the sky and see the Sun slowly being blocked out,* becoming a ominous black sphere that glows around the edges. Could you really be faulted for thinking that this was some sort of supernatural event, or that the end of the world was nigh?
Of course not. Which is why for thousands of years, human beings believed that solar eclipses were just that - a sign of death or a bad omen. But in fact, an eclipse is merely what happens when one stellar object passes in front of another and obscures it. In astronomy, this happens all the time; and between the Sun, the Moon, and the Earth, total eclipses have been witnessed countless times throughout history.
Definition: The general term for when one body passes in front of another in a solar system is transit. This term accurately describes how, depending on your vantage point, stellar bodies pass in front of each other on a regular basis, thus causing the reflected light from that body to be temporarily obscured.
https://youtu.be/Hn5nKlMY5cI
However, when we are talking about how the Moon can pass between the Earth and the Sun, and how the Earth can pass in front of the Moon, we use the term eclipse. This is also known as a syzygy, an astronomical term derived from ancient Greek (meaning "yoked together") that describes a straight-line configuration between three celestial bodies.
Total Solar Eclipse: When the Moon passes between the Sun and the Earth, and the Moon fully occults (blocks) the Sun, it is known as the solar eclipse. The type of solar eclipse event - total or partial - depends on the distance of the Moon from the Earth during the event.
During an eclipse of the Sun, only a thin path on the surface of the Earth is actually able to experience a total eclipse - this is called the path of totality. People on either side of that path see a partial eclipse, where the Sun is only partly obscured by the Moon, relative to those who are standing in the center and witnessing the maximum point of eclipse.
A total solar eclipse occurs when the Earth intersects the umbra portion - the innermost darkest part - of the Moon's shadow. These are relatively brief events, generally lasting only a few minutes, and can only be viewed along a relatively narrow track (up to 250 km wide). The region where a partial eclipse can be observed is much larger.
During a solar eclipse, the Moon can sometimes perfectly cover the Sun because its size is nearly the same as the Sun's when viewed from the Earth. This, of course, is an illusion brought on by the fact that the Moon is much closer to us than the Sun.
And since it is closer, it can block the light from the Sun and cast a shadow on the surface of the Earth. If you're standing within that shadow, the Sun and the Moon appear to line up perfectly, so that the Moon is completely darkened.
At that point, when the Sun is completely blocked by the Moon, you are witnessing what astronomers call a total solar eclipse - the point of maximum eclipse. After that, the Moon continues to move past the Sun, obscuring smaller and smaller portions of it and allowing more and more light to pass.
Total Lunar Eclipse: A total eclipse of the Moon is a different story. In this situation, the entire Moon passes into the Earth's shadow, darkening it fully. A partial lunar eclipse occurs when the shadow of the Earth doesn't fully cover the Moon, so only part of the Moon is darkened.
Unlike a solar eclipse, a lunar eclipse can be observed from nearly an entire hemisphere. In other words, observers all across planet Earth can see this darkening and it appears the same to all. For this reason it is much more common to observe a lunar eclipse from a given location. A lunar eclipse also lasts longer, taking several hours to complete, with totality itself usually averaging anywhere from about 30 minutes to over an hour.
There are three types of lunar eclipses. There's a penumbral eclipse, when the Moon crosses only the Earth's penumbra (the region in which only a portion of light is obscured); followed by a partial, when the Moon crosses partially into the Earth's umbra (the innermost and darkest part of a shadow, where the light is completely blocked).
Last, there is a total eclipse, when the Moon crosses entirely into the Earth's umbra. A total lunar eclipse involes the Moon passing through all three phases, then gradually admitting light again. Even during a total lunar eclipse, however, the Moon is not completely dark.
Sunlight is still refracted through the Earth's atmosphere and enters the umbra to provide faint illumination. Much as in a sunset, the atmosphere scatters shorter wavelength light, causing it to take on a red hue. This is where the phrase 'Blood Moon (http://www.universetoday.com/19969/red-moon/)' comes from.
https://youtu.be/ljwZMYy930s
Since the Moon orbits the Earth, you would expect to see an eclipse of the Sun once a lunar month, and an eclipse of the Moon once a lunar month. However, this does not happen simply because the Moon's orbit isn't lined up with the Sun. In fact, the Moon's orbit is tilted by a few degrees - 1.543º between the angle of the ecliptic and the lunar equator, to be exact.
This means that three objects only have the opportunity to line up and cause an eclipse a few times a year. It's possible for a total of 7 solar and lunar eclipses every year, but that only happens a few times every century.
Other Types of Eclipses: The term eclipse is most often used to describe either a solar eclipse or a lunar eclipse. However, it can also refer to such events beyond the Earth–Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its host planet, or a moon passing into the shadow of another moon.
For instance, during the Apollo 12 (http://www.nasa.gov/mission_pages/apollo/missions/apollo12.html) mission in 1969, the crew were able to observed the Sun being eclipsed by the Sun. In 2006, during its mission to study Saturn, the Cassini (https://saturn.jpl.nasa.gov/) spacecraft was able to capture the image below, which shows the gas giant transiting between it and the Sun.
In July of 2015, when the New Horizons (https://www.nasa.gov/mission_pages/newhorizons/main/) mission passed through the shadow of Pluto, it was able to capture a stunning image of the dwarf planet eclipsing the Sun. The image was taken at a distance of about 2 million km (1.25 million miles) which provided the necessary vantage point to fully obscure the disc of the Sun.
On top of that, many other bodies in the Solar System can experience eclipses as well. These include the four gas giants, all of which have major moons that periodically occult the planet, from the point of view of Earth-based observers.
The most impressive and common of these involve Jupiter and its four largest moons (Io, Europa, Ganymede and Callisto (http://www.universetoday.com/44796/galilean-moons/)). Given the size and low axial tilt of these moons, they often experience eclipses with Jupiter as a result of transiting relative to our observing instruments.
A well-known example occurred in April of 2014, when the Hubble Space Telescope caught an image of Ganymede passing in front atof Jupiter. At the time the image was taken, Ganymede was casting its shadow within Jupiter's Great Red Spot, which lent the planet a cyclops-like appearanc (http://www.universetoday.com/115759/just-in-time-for-halloween-jupiters-gets-a-giant-cyclops-eye/)e (see below).
The other three gas giants are known to experiences eclipses as well. However, these only occur at certain periods the planet's orbit of the Sun, due to their higher inclination between the orbits of their moons and the orbital plane of the planets. For instance, Saturn's largest moon Titan has been known to only occult the ringed gas giant once about every 15 years.
Pluto has also been known to experience eclipses with is largest moon (and co-orbiting body) Charon. However, in all of these cases, the eclipses are never total, as they do not have the size to obscure the much larger gas giant. Instead, the passage of the moons in front of the larger celestial bodies either cast small shadows on the cloud tops of the gas giants, or leads to an annular eclipse at most.
Similarly, on Mars, only partial solar eclipses are ever possible. This is because Phobos or Deimos are large enough (or distant enough in their orbits) to cover the Sun's disc, as seen from the surface of the planet. Phobos and Deimos have also been known to experience lunar eclipses as they slip into the shadow of Mars.
Martian eclipses have been photographed numerous times from both the surface and from orbit. For example, in 2010, the Spirit rover captured images of a Martian lunar eclipse (http://www.universetoday.com/11110/spirit-sees-a-martian-lunar-eclipse/) as Phobos, the larger of the two martian moons, was photographed while slipping into the shadow of Mars.
Also, between Nov. 4 and Nov. 5, 2010, the Opportunity rover captured several images (later turned into two movies) of a Martian sunset (http://www.universetoday.com/81885/videos-rover-captures-sunset-eclipse-on-mars/). In the course of imaging the Sun for a total of 17 minutes, Opportunity captured still of the Sun experiencing a Solar eclipse. On September 13th, 2012 - during the 37th day of its mission (Sol 27) - the Curiosity rover captured an image of Phobos transiting the Sun (http://www.universetoday.com/97408/curiosity-captures-a-martian-eclipse/).
As far as astronomical events go, total eclipses (Lunar and Solar) are not uncommon occurrences. If you ever want to witness a one, all you need to do is keep track of when one will be visible from your part of the world. A good resource for this is NASA's Eclipse Website (http://eclipse.gsfc.nasa.gov/eclipse.html) and timeanddate.com (https://www.timeanddate.com/eclipse/total-solar-eclipse.html).
Or, if you're the really adventurous type, you can find out where on Earth the next path of totality is going to be, and then book a vacation to go there. Get to the right spot at the right time, and you should be getting the view of a lifetime.
We have written many articles about the eclipse for Universe Today. Here's a list of articles (http://www.universetoday.com/tag/lunar-eclipse/) about specific times when a total Lunar Eclipse took place, and here's a list of Solar Eclipse (http://www.universetoday.com/tag/solar-eclipse/) articles. And be sure to check out this article and video of an Annular Eclipse (http://www.universetoday.com/95469/annular-eclipse-redux/).
If you'd like more info about the Eclipse, check out NASA Homepage (http://www.nasa.gov/), and here's a link to NASA's Solar System Simulator (http://space.jpl.nasa.gov/).
We've also recorded related episodes of Astronomy Cast about Eclipses. Listen here, Episode 160: Eclipses (http://www.astronomycast.com/2009/11/ep-160-eclipses/).
Sources:


NASA - Eclipse (http://eclipse.gsfc.nasa.gov/eclipse.html)
Wikipedia - Eclipse (https://en.wikipedia.org/wiki/Eclipse)
ESA - What is an Eclipse? (http://www.esa.int/Our_Activities/Space_Science/What_is_an_eclipse)
Time and Date - What are Solar Eclipses? (https://www.timeanddate.com/eclipse/solar-eclipse-frequency.html)

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