Space, Earth and Celestial Objects © Lisa Michalek

Celestial Objects The sun, moon, planets, stars or any distant object visible in the sky, generally appear to rise in the east and set in the west. The sun, moon, planets, stars or any distant object visible in the sky, generally appear to rise in the east and set in the west. Most appear to move along circular or curved paths. Most appear to move along circular or curved paths. The motion occurs at an angular rate of 15° per hour (360° in 24 hours). The motion occurs at an angular rate of 15° per hour (360° in 24 hours).

Motion of Stars and Planets Stars located over Earth’s equator follow nearly the same path as the sun. Stars located over Earth’s equator follow nearly the same path as the sun. Stars in the southern portion of the sky briefly arc across or appear over the horizon. Stars in the southern portion of the sky briefly arc across or appear over the horizon. Stars over the North Pole move in circles around Polaris. Stars over the North Pole move in circles around Polaris. In addition to their circular motion, planets change their positions from night to night. In addition to their circular motion, planets change their positions from night to night.

Constellations Although the stars are randomly distributed throughout the sky, ancient people imagined they saw patterns among them. Although the stars are randomly distributed throughout the sky, ancient people imagined they saw patterns among them. These patterns were often associated with traditions and legends that were part of the culture of early civilizations. These patterns were often associated with traditions and legends that were part of the culture of early civilizations. Some were said to resemble people, animals, or objects. Some were said to resemble people, animals, or objects.

Constellations

Regions in the Night Sky Astronomers have divided the night sky into 88 regions. Astronomers have divided the night sky into 88 regions. Each region is associated with a constellation. Each region is associated with a constellation.

Night Sky Models Early civilizations considered Earth to be a stationary object located at the center of the universe. Early civilizations considered Earth to be a stationary object located at the center of the universe. This was because they could not feel the Earth moving. This was because they could not feel the Earth moving. This early model of the universe was geocentric, or Earth-centered. This early model of the universe was geocentric, or Earth-centered.

Heliocentric Model When the paths of the planets were carefully measured by astronomers, some of the planetary motions were too complex and hard to explain. When the paths of the planets were carefully measured by astronomers, some of the planetary motions were too complex and hard to explain. Therefore astronomers support this more simple model. Therefore astronomers support this more simple model. This model includes all known planets revolving around the sun. This model includes all known planets revolving around the sun.

Heliocentric Model Includes two motions of planet Earth. Includes two motions of planet Earth. Each day Earth spins on its axis is known as rotation. Each day Earth spins on its axis is known as rotation. From west to east at the rate of 15° per hour (360° in 24 hours). From west to east at the rate of 15° per hour (360° in 24 hours). Earth also orbits the sun once per year known as revolution. Earth also orbits the sun once per year known as revolution. The earth takes 365 ¼ days to revolve around the sun. The earth takes 365 ¼ days to revolve around the sun. About 1° per day. About 1° per day.

Foucault Pendulum In 1851 the French scientist Jean Foucault suspended a long pendulum and set it swinging along a north-south line. In 1851 the French scientist Jean Foucault suspended a long pendulum and set it swinging along a north-south line. The Foucault Pendulum, mounted on a high support, is able to move in any direction. The Foucault Pendulum, mounted on a high support, is able to move in any direction. Foucault observed how the pendulum appeared to change direction as it swung freely in a clockwise direction. Foucault observed how the pendulum appeared to change direction as it swung freely in a clockwise direction. He interpreted this motion as the rotation of Earth under the pendulum. He interpreted this motion as the rotation of Earth under the pendulum.

Coriolis Effect Winds blowing out of a high-pressure system always curve to the right in the Northern Hemisphere. Winds blowing out of a high-pressure system always curve to the right in the Northern Hemisphere. The system of prevailing winds of Earth forms a series of symmetrical bands located north and south of the equator. The system of prevailing winds of Earth forms a series of symmetrical bands located north and south of the equator. The Coriolis effect is the result of inertia acting on a rotating planet. The Coriolis effect is the result of inertia acting on a rotating planet.

Motions of the Sun Before there were mechanical clocks, the movement of the sun through the sky was used to track the passage of time. Before there were mechanical clocks, the movement of the sun through the sky was used to track the passage of time. Sundials were among the first timekeeping devices used by humans. Sundials were among the first timekeeping devices used by humans. A sundial reveals the passage of the daylight hours as the shadow of a stick (gnomon) shifts across a marked dial or scale. A sundial reveals the passage of the daylight hours as the shadow of a stick (gnomon) shifts across a marked dial or scale.

Motions of the Sun Our system of time is based upon the apparent motions of the sun. Our system of time is based upon the apparent motions of the sun. The motions of celestial objects through the sky is called apparent motion because the object is not really moving as it appears to be. The motions of celestial objects through the sky is called apparent motion because the object is not really moving as it appears to be. The apparent rising and setting of the sun is actually caused by Earth’s rotation. The apparent rising and setting of the sun is actually caused by Earth’s rotation. The apparent solar day is the interval of time during which the sun passes from its highest point on one day to its highest point on the next. The apparent solar day is the interval of time during which the sun passes from its highest point on one day to its highest point on the next.

Seasons Because Earth moves in its orbit, the sun’s pathway from east to west in the sky also changes on a yearly cycle. Because Earth moves in its orbit, the sun’s pathway from east to west in the sky also changes on a yearly cycle. At the autumnal equinox (about Sept 23), the sun rises due east and sets due west. At the autumnal equinox (about Sept 23), the sun rises due east and sets due west. Daylight lasts for 12 hours, and it is the beginning of fall, or autumn, in the Northern Hemisphere. Daylight lasts for 12 hours, and it is the beginning of fall, or autumn, in the Northern Hemisphere. At the beginning of winter, the winter solstice (about Dec 21),the sun rises south of due east. At the beginning of winter, the winter solstice (about Dec 21),the sun rises south of due east. This is the shortest daylight period of the year. This is the shortest daylight period of the year.

Seasons At the vernal equinox (about March 21), the sun rises exactly in the east and sets due west. At the vernal equinox (about March 21), the sun rises exactly in the east and sets due west. The daylight period is 12 hours long, and it is the beginning of spring in the Northern Hemisphere. The daylight period is 12 hours long, and it is the beginning of spring in the Northern Hemisphere. When the summer solstice (about June 21), arrives the sun rises north of due east. When the summer solstice (about June 21), arrives the sun rises north of due east. The summer solstice is the longest daylight period of the year; it is the beginning of summer. The summer solstice is the longest daylight period of the year; it is the beginning of summer.

Seasons

Autumnal Equinox Autumnal Equinox Fall in the Northern hemisphere. Fall in the Northern hemisphere. Notice the amount of light and dark are equal in both hemispheres. Notice the amount of light and dark are equal in both hemispheres.

Winter Solstice Winter in the Northern hemisphere. Winter in the Northern hemisphere. Notice the North is getting much less daylight. Notice the North is getting much less daylight. Above the Arctic circle is in 24 hour darkness. Above the Arctic circle is in 24 hour darkness.

Vernal Equinox Vernal Equinox Spring in the Northern hemisphere. Spring in the Northern hemisphere. Both hemispheres are again getting the same number of hours of daylight. Both hemispheres are again getting the same number of hours of daylight. It is identical to the Autumnal Equinox. It is identical to the Autumnal Equinox.

Summer Solstice Summer Solstice Summer in the Northern hemisphere. Summer in the Northern hemisphere. Notice that the North is now getting many hours of daylight and above the Arctic circle is in 24 hour daylight. Notice that the North is now getting many hours of daylight and above the Arctic circle is in 24 hour daylight.

Latitude and the Angle of the Sun The path of the sun through the sky depends upon where you are when you observe it. The path of the sun through the sky depends upon where you are when you observe it. As you travel northward from any mid-latitude location, the noon sun will move lower and lower in the sky. As you travel northward from any mid-latitude location, the noon sun will move lower and lower in the sky. Observers at the poles see six months of daylight, followed by six months of darkness in a yearly cycle. Observers at the poles see six months of daylight, followed by six months of darkness in a yearly cycle. This is caused by a combination of the 23½° tilt of Earth’s axis and our revolution around the sun. This is caused by a combination of the 23½° tilt of Earth’s axis and our revolution around the sun.

Earth’s Axis

Daylight Hours At equinox, for all locations (except the poles), daylight lasts 12 hours. At equinox, for all locations (except the poles), daylight lasts 12 hours. At the equator throughout the year, there are 12 hours of daylight and 12 hours of night. At the equator throughout the year, there are 12 hours of daylight and 12 hours of night. As you approach the poles, the amount of seasonal variation in the length of daylight increases until a maximum six months of daylight and six months of darkness is reached at the North or South Pole. As you approach the poles, the amount of seasonal variation in the length of daylight increases until a maximum six months of daylight and six months of darkness is reached at the North or South Pole.

Daylight Hours