Motions in the sky: The celestial sphere

What is the celestial sphere? An imaginary sphere surrounding Earth that contains all the objects we see when we look at the sky. Imagine that there is a giant, spherical projector screen around the earth and all the light from objects in space is shining onto it to make the patterns we see (like constellations).

Star trails Why do stars appear to move across the sky?

Diurnal motion The entire sky appears to rotate once in 24 hours. This is termed the daily or diurnal motion of the celestial sphere, and results from the daily rotation of the earth on its axis. The diurnal motion affects all objects in the sky and does not change their positions relative to each other. Some objects also appear to move with respect to the other objects on the celestial sphere. These are the "wanderers" of the ancient astronomers: the planets, the Sun, and the Moon. This is due to the Earth’s orbit around the sun.

The Celestial Sphere We can use the “celestial sphere” idea to help locate objects in the sky. The stars rotate around the North and South Celestial Poles. These are the points in the sky directly above the Earth’s north and south poles. Fortunately, for those in the northern hemisphere, there is a fairly bright star very close to the North Celestial Pole called Polaris (or the North star). Another important reference marker is the celestial equator: an imaginary circle around the sky directly above the Earth's equator. All the stars rotate in a path that is parallel to the celestial equator.

Stars that are close to the celestial poles never appear to rise or set. These are called circumpolar stars.

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The "Road of the Sun" on the Celestial Sphere The sun has a diurnal (daily) motion from east to west due to the earth’s spinning around its axis, ~ 24 h The sun also changes its position in the sky ~ 1 degree per day, ~ 365.25 days

What is the ecliptic? The Earth and sun orbit each other in the same plane, called the ecliptic. That means the path of the sun across the sky will be on the ecliptic.

Constellations of the Zodiac Your zodiac “sign” is the constellation the sun was in on the day you were born. Since the Earth orbits the sun, when standing on Earth and looking up at the sun, there will be a different constellation in the background depending on the time of year.

Ecliptic and Zodiac Sun travels 360o/365.25 days ~ 1o/day Figure 3.1: The motion of Earth around the sun makes the sun appear to move against the background of the stars. The circular orbit of Earth is thus projected on the sky as the circular path of the sun, the ecliptic. Sun travels 360o/365.25 days ~ 1o/day

axis Celestial equator As a result, planes of the ecliptic and celestial equator make an angle 23.5o

The Solstices The solstices are the two spots farthest away from the celestial equator. Summer solstice: longest day of the year Winter solstice: shortest day of the year

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The Equinoxes The ecliptic and the celestial equator cross in two places. In these two spots, there is an equal amount of day and night (12 hours each). These are the equinoxes.

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The tilt of the Earth’s axis, NOT distance from the sun!!! What causes the seasons? The tilt of the Earth’s axis, NOT distance from the sun!!!

Tilt of the axis When the northern hemisphere is tilted towards the sun, we receive more direct sunlight. When we are tilted away from the sun, its rays are spread out over a larger area so they don’t give us as much warmth.

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Seasons - summary Seasons are NOT caused by varying distances from the Earth to the Sun The primary cause of seasons is the 23.5 degree tilt of the Earth's rotation axis with respect to the plane of the ecliptic. The Seasons in the Northern Hemisphere Note: the Earth is actually closest to the Sun in January 4!

Thus, we experience Summer in the Northern Hemisphere when the Earth is on that part of its orbit where the N. Hemisphere is oriented more toward the Sun and therefore: the Sun rises higher in the sky and is above the horizon longer, The rays of the Sun strike the ground more directly. Likewise, in the N. Hemisphere Winter the hemisphere is oriented away from the Sun, the Sun only rises low in the sky, is above the horizon for a shorter period, and the rays of the Sun strike the ground more obliquely.