Lunar Eclipse – September 27, 2015

This Sunday (September 27, 2015) much of  North America will be treated to a total lunar eclipse. The spectacle will be especially great because this will be a Supermoon eclipse. The term supermoon is popularly used to describe a full moon that occurs at or near the moon’s perigee, or closest point to earth in its orbit. The effect is that the moon will appear up to 14% larger and 33% brighter than normal.

Because this will be a total eclipse, the moon will take on a orange-reddish cast, popularly known as a Blood Moon.  So I guess one might say this will be a Super-Blood Moon!

What is a Lunar Eclipse?

Of course, the moon does not produce its own light. We all know that we see the moon because it reflects light from our sun. Occasionally the Earth will come between the sun and the moon and when it does it casts a shadow on the moon.

Credit: Geometry of a Lunar Eclipse - Sagredo

Credit: Geometry of a Lunar Eclipse – Sagredo

As you can see in the illustration above the earth’s shadow, like any shadow actually, has two sections of primary interest. The Penumbra is a partial shadow and occurs because only part of the light from the sun is being blocked. The Umbra is the region where the Earth completely blocks the sun. When the moon enters the Penumbra we call this a Penumbral eclipse, but it is difficult to detect to the casual observer. When the moon is partially under the Umbra we call this a Partial Eclipse and when it is fully inside the Umbra we call it a Total Eclipse.

So why is the moon in a total eclipse reddish-orange and not just blacked out?

When light passes from one medium into another it slightly changes its angle, this is called refraction.  When light passes by the sides of the Earth it passes through our atmosphere. Our atmosphere acts like a lens and causes the light to bend around the planet which allows it to shine on the moon.

So that explains why the moon is illuminated, but why is it reddish orange (a Blood Moon)? Light from the sun contains practically all wavelengths of visible light, from very short wavelength (high frequency) violet and blue light to very long wavelength (low frequency) orange and red light. Water vapor and dust in our atmosphere are big enough to block and scatter the very short wavelength violet and blue light which leaves the light passing through to the moon relatively more saturated with the orange and red end of the spectrum. The more dust in the atmosphere the redder the moon will appear during a total eclipse.

When to Watch

For a lunar eclipse there is no special safety equipment needed.  The moon is as safe to look at with bare eyes as it is any other night of the year. (Never look directly at a Solar Eclipse without special protective lenses.) The moon will rise already in the Earth’s penumbral shadow on Sunday, although it will not be obvious to the casual observer.

  • At 7:07 PM MDT (GMT -6) the moon will begin to enter the earth’s umbral shadow and the partial eclipse will begin.
  • At 8:11 PM MDT the moon will be fully eclipsed (under the Earth’s umbral shadow).
  • At 9:23 PM MDT the moon will begin to exit the umbral shadow and will again be in partial eclipse.
  • At 10:27 PM MDT the moon partial eclipse will end.

The penumbral eclipse ends at 11:22 PM MDT, but again, it will not be apparent to the casual observer.

Additional Information

See our Online Resources page and go to the link for information about The Moon and also see the information about Eclipses. don’t miss the handy Moon Phase Calendars under the link for Moon information.  Also, be sure to check out the link to Mr. for a wealth of information about this eclipse and those to come.

Eclipse fans should stay tuned to for upcoming information about the upcoming Total Solar Eclipse in North America in August 2017!

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