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Position for lunar eclipse Earth's
to occur when Earth is between surface
the Sun and the Moon Plane of
the Moon's
orbit Moon
Shadow
Sun Umbra
Penumbra
Earth Moon Plane of
Earth's
orbit

Position for solar eclipse
to occur when the Moon is FIGURE 16.32 People in a location where the tip of the um-
between Earth and the Sun bra falls on the surface of Earth see a total solar eclipse. People
in locations where the penumbra falls on the Earth’s surface see
FIGURE 16.31 The plane of the Moon’s orbit is inclined to the a partial solar eclipse.
plane of Earth’s orbit by about 5°. An eclipse occurs only where the
two planes intersect and Earth, the Moon, and the Sun are in a line.

close enough so at least the tip of its umbra reaches the surface
falls on Earth, people see a total solar eclipse. During a total of Earth. If the Moon’s umbra fails to reach Earth, an annular
solar eclipse, the new Moon completely covers the disk of the eclipse occurs. Annular means “ring-shaped,” and during this
Sun. The total solar eclipse is preceded and followed by a partial eclipse, the edge of the Sun is seen to form a bright ring around
eclipse, which is seen when the observer is in the penumbra. If the Moon. As before, people located in the area where the pen-
the observer is in a location where only the penumbra passes, umbra falls will see a partial eclipse. The annular eclipse occurs
then only a partial eclipse will be observed (Figure 16.32). More more frequently than the total solar eclipse.
people see partial than full solar eclipses because the penumbra When the Moon is full and the Sun, Moon, and Earth are
covers a larger area. The occurrence of a total solar eclipse is a lined up so Earth’s shadow falls on the Moon, a lunar eclipse
rare event in a given location, occurring once every several hun- occurs. Earth’s shadow is much larger than the Moon’s diameter,
dred years and then lasting for less than 7 minutes. so a lunar eclipse is visible to everyone on the night side of Earth.
The Moon’s cone-shaped shadow averages a length of This larger shadow also means a longer eclipse that may last for
375,000 km (about 233,000 mi), which is less than the average hours. As the umbra moves over the Moon, the darkened part
distance between Earth and the Moon. The Moon’s elliptical takes on a reddish, somewhat copper-colored glow from light
orbit brings it sometimes closer to and at other times farther refracted and scattered into the umbra by Earth’s atmosphere.
from Earth. A total solar eclipse occurs only when the Moon is This light passes through the thickness of Earth’s atmosphere on
its way to the eclipsed Moon, and it acquires the reddish color
for the same reason that a sunset is red: much of the blue light
TABLE 16.2 has been removed by scattering in Earth’s atmosphere.
Total eclipses in the United States, 2010–2025*
Total Solar Eclipses TIDES
If you live near or have ever visited a coastal area of the ocean,
Aug 21, 2017 Path: Oregon to South Carolina
you are familiar with the periodic rise and fall of sea level known
Apr 8, 2024 Path: Texas to Maine
as tides. The relationship between the motions of the Moon and
Total Lunar Eclipses the magnitude and timing of tides has been known and studied
since very early times. These relationships are that (1) the greatest
Dec 21, 2010 All visible
range of tides occurs at full and new moon phases, (2) the least
Dec 10, 2011 Visible at moonset
range of tides occurs at quarter moon phases, and (3) in most
Apr 15, 2014 Visible at moonset
oceans, the time between two high tides or between two low tides
Oct 8, 2014 All visible western; at moonset eastern
is an average of 12 hours 25 minutes. The period of 12 hours
Apr 4, 2015 Visible at moonset
25 minutes is one-half the average time interval between con-
Sept 28, 2015 Western visible at moonrise; all eastern secutive passages of the Moon across the celestial meridian. A
Jan 31, 2018 Visible at moonset location on the surface of Earth is directly under the Moon when
Jan 21, 2019 All visible it crosses the meridian and directly opposite it on the far side of
May 26, 2021 Visible at moonset Earth an average 12 hours 25 minutes later. There are two tidal
May 16, 2022 Visible moonrise western; all eastern bulges that follow the Moon as it moves around Earth, one on the
side facing the Moon and one on the opposite side. In general,
Mar 14, 2025 All visible
tides are a result of these bulges moving westward around Earth.
A simplified explanation of the two tidal bulges involves
*Eclipse predictions by Fred Espenak, NASA/GSFC.
See http://eclipse.gsfc.nasa.gov/eclipse.html. two basic factors: the gravitational attraction of the Moon and

424 CHAPTER 16 Earth in Space 16-20