Discovering the Universe Eighth Edition Discovering the Universe Eighth Edition Neil F. Comins William J. Kaufmann III CHAPTER 1 Discovering the Night Sky CHAPTER 1 Discovering the Night Sky

Cyclic motions of the Sun and stars in our sky are due to motions of the Earth 1.ROTATION = the spin of the Earth on its axis. It takes one day (24hrs) for the Earth to complete one rotation. If observing Earth from the north pole, the earth rotates counter-clockwise. 2.REVOLUTION = the movement of the Earth in orbit around the sun. It takes one year ( days) for the Earth to complete one revolution.

A sidereal month is the time it takes for the Moon to orbit the Earth with respect to the stars and is 27.3 days long. A synodic month is the time it takes for the Moon to orbit the Earth with respect to the Sun and is 29½ days long. The two times are different because the Earth moves in its orbit around the Sun as the Moon moves in its orbit around the Earth.

Another familiar cycle is the lunar cycle. When the Moon orbits the Earth, the amount of the side facing the Earth that is lit changes, creating the Moon’s phases. This phase cycle is called the synodic period and is 29½ days long.

Synchronous Rotation of the Moon The motion of the Moon around Earth as seen from above Earth’s north polar region (ignoring Earth’s orbit around Earth-Moon barycenter). For the Moon to keep the same side facing Earth as it orbits our planet, the Moon must rotate on its axis at precisely the same rate that it revolves around Earth.

One common misconception is that the Moon is only visible at night. However, the time of day in which the Moon is in our sky varies depending on its phase. This picture clearly displays the Moon, visible during the day.

During a new or full moon phase, when the Moon, Sun, and Earth are aligned, the Moon may enter the shadow of the Earth, or the shadow of the Moon may reach the Earth, creating eclipses. However, these eclipses do not occur during every full or new moon because the Moon’s orbit is tilted by 5  with respect to the Earth-Sun (ecliptic) plane.

Types of Lunar Eclipses: PENUMBRAL = the Moon appears dimmed. PARTIAL = part of the Moon enters the umbra of the Earth’s shadow and is darkened. TOTAL = all of the Moon enters the Earth’s shadow and becomes a reddish color, only lit from light bending around the Earth’s atmosphere.

During a total lunar eclipse, the Moon moves in and out of the umbra of the Earth’s shadow.

Solar Eclipses

Solar Eclipses: Solar Eclipses: If you are located where the umbra of the Moon’s shadow reaches, you will see a total solar eclipse, during which the entire disk of the Sun is covered by the Moon, revealing the faint solar corona surrounding the Sun. If you are located where the umbra of the Moon’s shadow reaches, you will see a total solar eclipse, during which the entire disk of the Sun is covered by the Moon, revealing the faint solar corona surrounding the Sun.

Unlike lunar eclipses, solar eclipses occur at specific places on the Earth, indicated by the arrow. Partial Solar Eclipse: occurs in places covered by the penumbra of the Sun’s shadow

Annular eclipses: Sometimes eclipses occur when the Moon is too far away from the Earth to completely cover the Sun in our sky. When this occurs, the Moon appears in the center and a thin ring, or “annulus,” of light surrounds it. These are called annular eclipses. watch?v=CTbIufApsSk

Eclipse Paths for Total and Annular Eclipses 2001– This map shows the eclipse paths for the 14 total solar and 13 annular eclipses that occur between 2001 and In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across the Earth’s surface.

Summary of Key Ideas

Patterns of Stars The surface of the celestial sphere is divided into 88 unequal areas called constellations. 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. The boundaries of the constellations run along lines of constant right ascension or declination.

Earthly Cycles The celestial sphere appears to revolve around the Earth once in each day-night cycle. In fact, it is the 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 the Earth out onto the celestial sphere.

Earthly Cycles Earth’s axis of rotation is tilted at an angle of 231⁄2° from the perpendicular to the plane of the Earth’s orbit (the plane of the ecliptic). This tilt causes the seasons. Equinoxes and solstices are significant points along the Earth’s orbit that are determined by the relationship between the Sun’s path on the celestial sphere (the ecliptic) and the celestial equator.

The 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 Moon on the Earth’s equatorial bulge. The 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 Moon on the Earth’s equatorial bulge. The length of the day is based upon the Earth’s rotation rate and the average motion of the Earth around the Sun. These effects combine to produce the 24-hour day upon which our clocks are based. The length of the day is based upon the Earth’s rotation rate and the average motion of the Earth around the Sun. These effects combine to produce the 24-hour day upon which our clocks are based. Earthly Cycles

The phases of the Moon are caused by the relative positions of the Earth, Moon, and Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days. The phases of the Moon are caused by the relative positions of the Earth, Moon, and Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days. The Moon completes one orbit around the Earth with respect to the stars in a sidereal month, which averages 27.3 days. The Moon completes one orbit around the Earth with respect to the stars in a sidereal month, which averages 27.3 days. Earthly Cycles

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. 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 the Earth’s shadow. During a lunar eclipse, the Sun, Earth, and Moon are in alignment with the Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic. A lunar eclipse occurs when the Moon moves through the Earth’s shadow. During a lunar eclipse, the Sun, Earth, and Moon are in alignment with the Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic.

Eclipses A solar eclipse occurs when a strip of the Earth passes through the Moon’s shadow. During a solar eclipse, the Sun, Earth, and Moon are in alignment with the Moon between the Earth and the Sun, and the Moon is in the plane of the ecliptic. A solar eclipse occurs when a strip of the Earth passes through the Moon’s shadow. During a solar eclipse, the Sun, Earth, and Moon are in alignment with the Moon between the Earth and the Sun, and the Moon is in the plane of the ecliptic. Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total. Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total.

angle angular diameter (angular size) annular eclipse arc angle celestial equator celestial pole celestial sphere circumpolar star constellation declination degree diurnal motion eclipse path ecliptic equinox gravitation line of nodes lunar eclipse lunar phase north celestial pole partial eclipse penumbra penumbral eclipse precession precession of the equinoxes revolution right ascension rotation scientific notation sidereal month sidereal period solar corona solar day solar eclipse south celestial pole summer solstice synodic month terminator time zone total eclipse umbra vernal equinox winter solstice zenith zodiac Key Terms

WHAT DID YOU THINK? Is the North Star—Polaris—the brightest star in the night sky? 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. No. Polaris is a star of medium brightness compared with other stars visible to the naked eye.

WHAT DID YOU THINK? Do astronomers regard constellations as the familiar patterns of stars in the sky? Do astronomers regard constellations as the familiar patterns of stars in the sky? 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. 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.

WHAT DID YOU THINK? What causes the seasons? What causes the seasons? The tilt of the Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from the Earth to the Sun that results from the shape of Earth’s orbit. The tilt of the Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from the Earth to the Sun that results from the shape of Earth’s orbit.

WHAT DID YOU THINK? When is the Earth closest to the Sun? On or around January 3 of each year.

WHAT DID YOU THINK? How many zodiac constellations are there? There are 13 zodiac constellations, the lesser-known one being Ophiuchus.

WHAT DID YOU THINK? 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.