Eclipse Details: Lunar Eclipse

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Let's explore a little more about lunar and solar eclipses. Remember that an eclipse happens when an object passes through another object's shadow. Any shadow consists of two parts: an umbra which is the region of total shadow and the penumbra which is the outer region of partial shadow. If the Moon were to pass through the Earth's umbra, an observer on the Moon would not be able to see the Sun at all---she would observe a solar eclipse! An observer on the Earth looking at the Moon would see a total lunar eclipse. The Earth's shadow is pretty big compared to the Moon so a total lunar eclipse can last up to about 1 hour 45 minutes.

If the Moon only passed through the outer part of the shadow (the penumbra), then the observer on the Moon would see the Sun only partially covered up---a partial solar eclipse. The observer on the Earth would see the Moon only partially dimmed---a partial lunar eclipse. The total lunar eclipses of 2014 are shown below.
Lunar eclipses of 2014

total lunar eclipse of April 2014

Lunar eclipse of April 2014 -- the bright star to the right of the Moon is Spica.

lunar eclipse of October 2014

Lunar eclipse of October 2014

Below is a sequence of images from the August 28, 2007 lunar eclipse taken with a digital camera at the highest zoom setting. The dark red-orange color is the color I saw. Click the box to pause the movie. Click the right arrow to play the movie.

During a total lunar eclipse you see another interesting effect---the Moon turns a coppery (or bloody) red. The reason why some sunlight reaches the Moon despite the fact that the Moon is in the Earth's umbra is that the sunlight refracts or bends as it passes through the Earth's atmosphere. Dust particles in the Earth's atmosphere remove much of the bluer colors in the sunlight so only the redder colors make it to the Moon. The amount of dust determines the deepness of the red colors. The dust in the air is also why the Sun appears redder at sunset on Earth. The observer on the Moon would see a reddish ring around the Earth even at mid-eclipse! Some nice visualizations of the lunar eclipse essentials are available at Goddard Space Flight Center's Lunar Eclipse Essentials page.

lunar eclipse geometry

Eclipse Details: Solar Eclipse

The Moon's shadow also has an umbra and penumbra, but its shadow is much smaller than the Earth's. Only if the Moon is in the ecliptic plane when it is exactly New Moon will the Moon's shadow hit the Earth. Where the umbra hits the Earth, you will see a total solar eclipse. Where the penumbra hits the Earth, you will see a partial solar eclipse. In a total solar eclipse the bright disk of the Sun is completely covered up by the Moon and you can see the other parts of the Sun like the corona, chromosphere, and prominences as shown in the images below.

total solar eclipse of August 21, 2017

total solar eclipse of August 21, 2017

Select the images to view full-size versions. The first total solar eclipse image above is a combination of exposures in an attempt to capture the fuller range of what the human eye could see but, even still, the prominences were easier to see than what the image shows. The second total solar eclipse figure shows two of the exposures used to create the first image. The short exposure image on the left makes it easier to see the prominences sticking out from behind the 12 o'clock, 1:30 o'clock, and 3 o'clock edges of the dark Moon.

Unfortunately, only the tip of the Moon's umbra reaches the Earth (the tip hitting the Earth is at most 270 kilometers [168 miles] in diameter) and it zips along the Earth's surface at over 1600 kph (1000 mph) as the Moon moves around the rotating Earth. This means that a total solar eclipse can last a maximum of only 7.5 minutes. Usually total solar eclipses last only 2-3 minutes. Because of the orbital motion of the Moon and the rotation of the Earth, the umbra makes a long, narrow path of totality. For example, the August 21, 2017 eclipse had a narrow path of totality about 109 kilometers (68 miles) wide from the northwest coast to the southeast coast of the United States as shown below. How this looked from space is available from this link to an MP4 video of EPIC images from the DSCOVR satellite (source link and annotated version on YouTube).

NASA's eclipse map for August 21, 2017

side view of Moon orbit 
with shadows

Sometimes the umbra does not reach the Earth at all (only the penumbra) even though the Moon is on the ecliptic and it is exactly in New Moon phase. A bright ring will be visible around the Moon when it is lined up with the Sun---an annular eclipse (because of the annulus or ring of light around the Moon). What do you think this implies about the shape of the Moon's orbit?

why annular eclipses occur

The sequence below is of the May 20, 2012 annular solar eclipse as seen from Red Bluff, CA. Select the image to go to a full-size image of the eclipse sequence and an animation.

Annular eclipse of May 20, 2012

A video of the central 5 minutes of the total solar eclipse of November 13, 2012 as seen from Amaroo, outside of Cairns, Australia, is posted on YouTube. The video also shows that even astronomers have to contend with the weather. A video of images from the August 21, 2017 total solar eclipse as seen from Glendo State Park in Wyoming, USA is posted on YouTube. The video shows the Moon covering the Sun, different exposures during totality to highlight different parts of the eclipsed Sun, and the Moon uncovering the Sun. Another video of a wide-angle view of the August 2017 total solar eclipse shows the changing sky color and eclipsed Sun in real-time.

Eclipse Web Sites

The following sites are probably the best and most complete ones on the web. You'll find eclipse predictions, photographs, observing instructions, and fairly complete indexes of all high-quality eclipse sites on the web. Between these three you should find whatever you may be interested in with regard to eclipses. They will display in another window.
  1. NASA's Eclipse Bulletins.
  2. Fred Espenak's eclipse site. A less technical version of the NASA site. Fred Espenak is also the main contributer to the NASA site.
  3. Sky & Telescope's eclipse site.

Vocabulary

annular eclipse penumbra refraction
umbra

Review Questions

  1. What are the two parts of a shadow and in which part is the Sun partially visible?
  2. Why does a new phase occur every month but a lunar eclipse occurs only twice a year?
  3. Why is the path of totality of a solar eclipse different each time? Why does the latitude of the path vary?
  4. Why does the Moon turn orange-red during a total lunar eclipse?
  5. Would a person on the Moon ever experience an annular solar eclipse? Explain your answer.
  6. Why do we not have just annular solar eclipses or just total solar eclipses when the Moon and Sun are exactly lined up?

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last updated: August 26, 2017

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel