Solar Eclipse


What is an eclipse of the Sun? What causes eclipses and why? How often do eclipses happen and when is the next eclipse of the Sun? You’ll learn the answers to these questions and more in MrEclipse’s primer on solar eclipses. Before we learn more about the eclipses of the Sun, we need to first talk about the Moon.

Phases of The Moon

The Moon is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own but shines by sunlight reflected from its surface. The Moon orbits Earth about once every 29 and a half days. As it circles our planet, the changing position of the Moon with respect to the Sun causes our natural satellite to cycle through a series of phases:

      • New Moon > New Crescent > First Quarter > Waxing Gibbous > Full Moon >
        Waning Gibbous > Last Quarter > Old Crescent > New Moon (again)

The phase known as New Moon can not actually be seen because the illuminated side of the Moon is then pointed away from Earth. The rest of the phases are familiar to all of us as the Moon cycles through them month after month. Did you realize that the word month is derived from the Moon’s 29.5 day period?

To many early civilizations, the Moon’s monthly cycle was an important tool for measuring the passage of time. In fact many calendars are synchronized to the phases of the Moon. The Hebrew, Muslim and Chinese calendars are all lunar calendars. The New Moon phase is uniquely recognized as the beginning of each calendar month just as it is the beginning on the Moon’s monthly cycle. When the Moon is New, it rises and sets with the Sun because it lies very close to the Sun in the sky. Although we cannot see the Moon during New Moon phase, it has a very special significance with regard to eclipses.

Geometry of the Sun, Earth and Moon During an Eclipse of the Sun
The Moon’s two shadows are the penumbra and the umbra.
(Sizes and distances not to scale)

The Moon’s Two Shadows

An eclipse of the Sun (or solar eclipse) can only occur at New Moon when the Moon passes between Earth and Sun. If the Moon’s shadow happens to fall upon Earth’s surface at that time, we see some portion of the Sun’s disk covered or ‘eclipsed’ by the Moon. Since New Moon occurs every 29 1/2 days, you might think that we should have a solar eclipse about once a month. Unfortunately, this doesn’t happen because the Moon’s orbit around Earth is tilted 5 degrees to Earth’s orbit around the Sun. As a result, the Moon’s shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon’s shadow falls on Earth’s surface and an eclipse of the Sun is seen from that region.

The Moon’s shadow actually has two parts:

  1. Penumbra     
        • The Moon’s faint outer shadow.
        • Partial solar eclipses are visible from within the penumbral shadow.
    1. Umbra
        • The Moon’s dark inner shadow.
        • Total solar eclipses are visible from within the umbral shadow.

    When the Moon’s penumbral shadow strikes Earth, we see a partial eclipse of the Sun from that region. Partial eclipses are dangerous to look at because the un-eclipsed part of the Sun is still very bright. You must use special filters or a home-made pinhole projector to safely watch a partial eclipse of the Sun.

    What is the difference between a solar eclipse and a lunar eclipse? A lunar eclipse is an eclipse of the Moon rather than the Sun. It happens when the Moon passes through Earth’s shadow. This is only possible when the Moon is in the Full Moon phase.



    Total Solar Eclipse and Path of Totality

    Total Solar Eclipses and the Path of Totality

    If the Moon’s inner or umbral shadow sweeps across Earth’s surface, then a total eclipse of the Sun is seen. The track of the Moon’s umbral shadow across Earth is called the Path of Totality. It is typically 10,000 miles long but only about 100 miles wide. It covers less than 1% of Earth’s entire surface area. In order to see the Sun become completely eclipsed by the Moon, you must be somewhere inside the narrow path of totality.

    The path of a total eclipse can cross any part of Earth. Even the North and South Poles get a total eclipse sooner or later. Just one total eclipse occurs each year or two. Since each total eclipse is only visible from a very narrow track, it is rare to see one from any single location. You’d have to wait an average of 375 years to see two total eclipses from one place. Of course, the interval between seeing two eclipses from one particular place can be shorter or longer. For instance, the last total eclipse visible from Princeton, NJ was in 1478 and the next is in 2079. That’s an interval of 601 years. However, the following total eclipse from Princeton is in 2144, after a period of only 65 years.

    2006 Total Solar Eclipse
    A composite image reveals subtle structure in the Sun’s corona.

    Awesome Totality

    The total phase of a solar eclipse is very brief. It rarely lasts more than several minutes. Nevertheless, it is considered to be one of the most awe inspiring spectacles in all of nature. The sky takes on an eerie twilight as the Sun’s bright face is replaced by the black disk of the Moon. Surrounding the Moon is a beautiful gossamer halo. This is the Sun’s spectacular solar corona, a super heated plasma two million degrees in temperature. The corona can only be seen during the few brief minutes of totality. To witness such an event is a singularly memorable experience which cannot be conveyed adequately through words or photographs. Nevertheless, you can read more about the Experience of Totality in the first chapter of Totality – Eclipses of the Sun.


    Scientists welcome the total eclipse as a rare opportunity to study the Sun’s faint corona. Why is the corona so hot? What causes it to spew massive bubbles of plasma into space through coronal mass ejections? Can solar flares be predicted and what causes them? These major mysteries may eventually be solved through experiments performed at future total eclipses.


    For amateur astronomers and eclipse chasers, an eclipse of the Sun presents a tempting target to photograph. Fortunately, Solar Eclipse Photography is easy provided that you have the right equipment and use it correctly. See MrEclipse’s Picks for camera, lens and tripod recommendations. For more photographs taken during previous lunar eclipses, be sure to visit Solar Eclipse Photo Gallery. It’s also possible to capture a solar eclipse using a video camcorder.


    The total solar eclipse occurred on March 29, 2006 and was visible from Africa and central Asia. Fred Espenak led a Spears Travel tour to Libya to witness the event. You can see a collection of his photographs at 2006 Eclipse Gallery. Reports (with photos) from some of his earlier eclipse expeditions include 2001 Eclipse in Zambia, 1999 Eclipse in Turkey, 1998 Eclipse in Aruba and 1995 Eclipse in India.


    The next two total eclipse of the Sun occur on: March 20, 2015 and March 09, 2016. Join Fred Espenak on a Spears Travel tour to witness one (or both!) of these spectacular events.

    Annular Solar Eclipse and the Path of Annularity

    Annular Solar Eclipses

    Unfortunately, not every eclipse of the Sun is a total eclipse. Sometimes, the Moon is too small to cover the entire Sun’s disk. To understand why, we need to talk about the Moon’s orbit around Earth. That orbit is not perfectly round but is oval or elliptical in shape. As the Moon orbits our planet, its distance varies from about 221,000 to 252,000 miles. This 13% variation in the Moon’s distance makes the Moon’s apparent size in our sky vary by the same amount. When the Moon is on the near side of its orbit, the Moon appears larger than the Sun. If an eclipse occurs at that time, it will be a total eclipse. However, if an eclipse occurs while the Moon is on the far side of its orbit, the Moon appears smaller than the Sun and can’t completely cover it. Looking down from space, we would see that the Moon’s umbral shadow is not long enough to reach Earth. Instead, the antumbra shadow reaches Earth.

    The track of the antumbra is called the path of annularity. If you are within this path, you will see an eclipse where a ring or annulus of bright sunlight surrounds the Moon at the maximum phase. Annular eclipses are also dangerous to look directly with the naked eye. You must use the same precautions needed for safely viewing a partial eclipse of the Sun

    Annularity can last as long as a dozen minutes, but is more typically about half that length. Since the annular phase is so bright, the Sun’s gorgeous corona remains hidden from view. But annular eclipses are still quite interesting to watch. You can read reports about the annular eclipses of 1999 in Australia, 2003 in Iceland, and 2005 in Spain. More recently, visit the 2012 Annular Solar Eclipse Photo Gallery.




2005 Annular Solar Eclipse
This sequence shows the eclipse just before, during and after annularity.

2006 Total Solar Eclipse
This Baily’s Beads sequence shows both 2nd and 3rd Contact


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