Eclipses are among the most spectacular natural phenomena. During a lunar eclipse, the brilliant full Moon often darkens to a rusty red. A lunar eclipse occurs when the Moon passes through Earth’s shadow. This can happen only when the Sun, Earth, and the Moon are in a straight line at full Moon. During a solar eclipse, broad daylight is transformed into an eerie twilight, as the Sun seems to be blotted from the sky. A solar eclipse occurs when the Moon’s shadow moves across Earth’s surface. As seen from Earth, the Moon moves in front of the Sun—
At first glance, it would seem that eclipses should happen at every new and full Moon, but in fact, they occur much less often because the Moon’s orbit is tilted 5° from the plane of the ecliptic (Figure 1-23). Consequently, the new Moon and full Moon usually occur when the Moon is either above or below the plane of Earth’s orbit. In such positions, a true alignment between the Sun, Moon, and Earth is not possible. Hence, Earth and the Moon do not enter each others’ shadows and an eclipse cannot occur. This tilt also explains why, as mentioned earlier, the new Moon is often seen as a thin crescent—
Indeed, because its orbit is tilted 5° from the ecliptic, the Moon is usually above or below the plane of our orbit around the Sun. The Moon crosses the ecliptic at what is called the line of nodes (see Figure 1-
19
Earth’s shadow has two distinct parts, as shown in Figure 1-24a. The umbra is the part of the shadow where all direct sunlight is blocked by Earth. If you were in Earth’s umbra looking at Earth, you would not see the Sun behind it at all. The penumbra of the shadow is where Earth blocks only some of the sunlight. If you were in Earth’s penumbra looking at Earth, you would see a crescent Sun behind it. The Moon has an analogous umbra and penumbra.
Depending on how the Moon travels through Earth’s shadow, one of three kinds of lunar eclipse may occur. A penumbral eclipse, when the Moon passes through only Earth’s penumbra, is easy to miss. The Moon still looks full, just a little dimmer than usual and sometimes slightly reddish in color (path 1 in Figure 1-
When just part of the lunar surface passes through the umbra, a bite seems to be taken out of the Moon, and we see a partial eclipse (path 2 in Figure 1-
Date |
Visible from |
Type |
Duration of totality (h:min) |
---|---|---|---|
2015 Apr 04 |
Asia, Australia, Pacific, Americas |
Total |
:05 |
2015 Sep 28 |
Europe, Africa, west Asia, Americas |
Total |
1:12 |
2016 Mar 23 |
Asia, Australia, Pacific, western Americas |
Penumbral |
|
2016 Sep 16 |
Europe, Africa, Asia, Australia |
Penumbral |
|
2017 Feb 11 |
Europe, Africa, Asia, Americas |
Penumbral |
|
2017 Aug 07 |
Europe, Africa, Asia, Australia |
Partial |
|
2018 Jan 31 |
Asia, Australia, western North America |
Total |
1:16 |
2018 Jul 27 |
South America, Europe, Africa, Asia, Aus. |
Total |
1:43 |
20
Why does the new Moon sometimes appear as a crescent?
Even during a total eclipse, the Moon does not completely disappear. A small amount of sunlight passing through Earth’s atmosphere is bent into Earth’s umbra. The light deflected into the umbra is primarily red and orange, and thus the darkened Moon glows faintly in rust-
The different physical diameters of the Sun and Moon and their different distances from us combine so that they both have nearly the same angular diameter as seen from Earth—
Viewing the Sun directly for more than a moment at any time without an approved filter causes permanent eye damage. It is only safe to look at a total solar eclipse without a filter during the brief time when the Moon completely blocks the Sun. At all other times during the eclipse, you must view it either through an approved filter or in the image made on a flat surface by a telescope or a pinhole camera.
21
Earth’s rotation and the orbital motion of the Moon cause the umbra to race along the eclipse path at speeds in excess of 1700 km/h (1050 mi/h). For this reason, complete blockage of the Sun (totality) during a total eclipse never lasts for more than 7½ minutes at any one location on the eclipse path, and it usually lasts for only a few moments.
The Moon’s umbra is also surrounded by a penumbra (see Figure 1-
The Moon’s orbit around Earth is oval shaped. Therefore, the distance between Earth and the Moon, which averages 384,400 km (238,900 mi), varies by 14% as the Moon moves around Earth. Consequently, the angular size of the Moon in our sky varies by that amount. The width of the eclipse path depends primarily on the Earth–
Why aren’t there any annular lunar eclipses?
If what would otherwise be a total solar eclipse occurs when the Moon is relatively far from Earth, then the Moon’s umbra falls short of Earth and no one sees a total eclipse. From Earth’s surface, the Moon then appears too small to cover the Sun completely, and a thin ring or “annulus” of light is seen around the edge of the Moon at mid-
Date |
Type |
Visible from |
Total/annular eclipse time (min:sec) |
---|---|---|---|
2015 Mar 20 |
Partial/Total |
Partial: Iceland, Europe, northern Africa, northern Asia Total: north Atlantic Ocean |
2:47 |
2015 Sep 13 |
Partial |
Southern Africa, Antarctica |
|
2016 Mar 09 |
Partial/Total |
Partial: eastern Asia, Australia Total: Pacific Ocean, Borneo |
4:09 |
2016 Sep 01 |
Annular |
Atlantic Ocean, central Africa, Indian Ocean |
3:06 |
2017 Feb 26 |
Annular |
Pacific Ocean, Chile, Argentina, Atlantic Ocean, Africa |
0:44 |
2017 Aug 21 |
Partial/Total |
Partial: northern South America, all of North America except United States Total: United States |
2:40 |
2018 Feb 15 |
Partial |
southern South America |
|
2018 Jul 13 |
Partial |
southern Australia |
|
2018 Aug 11 |
Partial |
northern Europe, northeast Asia |
|
22
23