SUMMARY OF KEY IDEAS
Navigating the Night Sky
The surface of the celestial sphere is divided into 88 unequal areas called constellations.
The boundaries of the constellations run along lines of constant right ascension or declination, which are perpendicular to each other.
Earthly Cycles
The celestial sphere appears to revolve around Earth once in each day-
night cycle. In fact, it is Earth’s rotation that causes this apparent motion. The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of Earth out onto the celestial sphere.
The Seasons
Earth’s axis of rotation is tilted at an angle of 23½° from a line perpendicular to the plane of Earth’s orbit around the Sun (the plane of the ecliptic). This tilt causes the seasons.
Equinoxes and solstices are significant points along Earth’s orbit that are determined by the relationship between the Sun’s path on the celestial sphere (the ecliptic) and the celestial equator.
Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years, a phenomenon called precession. Precession is caused by the gravitational pull of the Sun and the Moon on Earth’s equatorial bulge.
The length of the day is based on Earth’s rotation rate and the average motion of Earth around the Sun. These effects combine to produce the 24-
hour day.
The Phases of the Moon
The phases of the Moon are caused by the relative positions of Earth, the Moon, and the Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days.
The Moon completes one orbit around Earth as measured with respect to the stars in a sidereal month, which averages 27.3 days.
Eclipses
The shadow of an object has two parts: the umbra, where direct light from the source is completely blocked; and the penumbra, where the light source is only partially obscured.
A lunar eclipse occurs when the Moon moves through Earth’s shadow. During a lunar eclipse, the Sun, Earth, and the Moon are in alignment, with Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic.
A solar eclipse occurs when a strip of Earth passes through the Moon’s umbra or penumbra. During a solar eclipse, the Sun, Earth, and the Moon are in alignment, with the Moon between Earth and the Sun, and the Moon is in the plane of the ecliptic.
Depending on the relative positions of the Sun, the Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total.
Scales of the Universe
Astronomy examines objects that range in size from the parts of an atom (∼10−15 m) to the size of the observable universe (∼1026 m).
Scientific notation is a convenient shorthand for writing very large and very small numbers.
WHAT DID YOU THINK?
What do astronomers define as constellations? Astronomers sometimes use the common definition of a constellation as a pattern of stars. Formally, however, a constellation is an entire area of the celestial sphere and all the stars and other objects in it. Viewed from Earth, the entire sky is covered by 88 different-sized constellations. If there is any room for confusion, astronomers refer to the patterns as asterisms. 
Is the North Star—Polaris— the brightest star in the night sky? No. Polaris is a star of medium brightness compared with other stars visible to the naked eye. 
What causes the seasons? The tilt of Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from Earth to the Sun that results from the oval shape of Earth’s orbit.
When is Earth closest to the Sun? Earth is closest to the Sun on or around January 3 of each year.
How many zodiac constellations are there? There are 13 zodiac constellations, the least-known one being Ophiuchus. 
Does the Moon have a dark side that we never see from Earth? Half of the Moon is always dark. Whenever we see less than a full Moon, we are seeing part of the Moon’s dark side. So, the dark side of the Moon is not the same as the far side of the Moon, which we never see from Earth.
Is the Moon ever visible during the daytime? The Moon is visible at some time during daylight hours almost every day of the year. Different phases are visible during different times of the day.
What causes lunar and solar eclipses? When the Moon is crossing the ecliptic in the full or new phase, the shadows of Earth or the Moon, respectively, then fall on the Moon or Earth. These shadows on the respective surfaces are eclipses.
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