CHAPTER 1: Discovering the Night Sky

Astronomers describe the universe as an imaginary sphere surrounding the earth on which all objects in the sky can be located, called the CELESTIAL SPHERE. As viewed from Earth, the celestial sphere appears to rotate arouns two axis points, the north and south celestial poles, which are located directly above the Earth’s poles. Between these is the celestial equator, which divides the celestial sphere into northern and southern hemispheres. We define the position of an object on the celestial sphere using two coordinates, right ascension and declination.

Cyclic motions of the Sun and stars in our sky are due to motions of the Earth ROTATION=the spin of the Earth on its axis. It takes one day for the Earth to complete one rotation. REVOLUTION=the movement of the Earth in orbit around the sun. It takes one year for the Earth to complete one revolution. PRECESSION=the slow conical (top-like) motion of the Earth’s axis of rotation. It takes 26,000 years for the Earth to complete one cycle of precession.

The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation. At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, they disappear somewhere along the western horizon. We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”

However, not all stars in our sky rise and set, and different latitudes view the movements of the stars differently. The view of the stars movement depends on our latitude. The stars near the poles of the celestial sphere (shown here) move in trails that circle the pole and never set. They are called circumpolar. NORTH POLE EQUATOR MIDDLE LATITUDES

Different parts of the world experience different times of day as the Earth rotates. TIME ZONES which can be used to calculate the time of day in any given part of the world.

The Earth also revolves around the Sun, which changes our view of the stars. From our perspective, the Sun appears to move through the stars along a special path called the ecliptic. From an outside view, we see the Earth revolve around the Sun. We define the plane of the Earth’s orbit as the ecliptic plane.

We can see how different stars appear at different times of day by looking at the position of the Sun against the backdrop of stars. The side of the Earth facing the Sun is experiencing “day,” while the side of the Earth turned away from the Sun is experiencing “night.” MARCH SEPTEMBER

Seasons are caused because the Earth’s axis is tilted and as the Earth revolves around the Sun, different parts of the Earth receive more direct sunlight (summer), while other parts of the Earth receive sunlight which is more spread out (winter).

The seasons we experience are linked to the motion of the Sun along the celestial sphere. The point of the Sun’s path farthest north on the celestial sphere is called the summer solstice (JUN 21), while the point of the ecliptic farthest south is called the winter solstice (DEC 21). The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 21), the Sun is moving north, while during the autumnal equinox (SEPT 21), the Sun is moving south. Remember that the seasonal names of the equinoxes and the solstices refer to seasons in the NORTHERN hemisphere. The seasons occurring in the SOUTHERN hemisphere are exactly opposite.

The Sun rises at different points along the horizon at different times of the year and also peaks at different heights.

Another familiar cycle is the lunar cycle 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.

One common misconception is that the Moon is only visible at night 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.

THE THREE 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.

A Total Lunar Eclipse

A Total Lunar Eclipse A total lunar eclipse can last up to 1 hour and 40 min. During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere.

SOLAR ECLIPSES occur when the moon’s shadow reaches the earth. Unlike lunar eclipses, solar eclipses occur at specific places on the Earth, indicated by the arrow.

Solar Eclipses The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon. When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.

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. Those just outside of this region where the penumbra of the Moon’s shadow reaches will only see a partial solar eclipse, during which only part of the disk of the Sun is covered by the Moon. 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. TOTAL ECLIPSE ANNULAR ECLIPSE

Total Solar Eclipse

Conditions for Eclipses . A solar eclipse can only occur if the moon passes a node near new moon. A lunar eclipse can only occur if the moon passes a node near full moon.