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CH. 1: CYCLES OF THE SKY
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1.1 The Celestial Sphere
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The nearest star to us is about 4 ly away. The celestial sphere is a model used to simplify our viewing of celestial bodies. The Stars in a constellation generally have no physical relation to one another, they re just in the same area of the sky.
Figure 1.1
Since the Earth spins, the stars move across the sky from East to West, just as the Sun does. Directly above our North and South poles are the celestial poles. The stars appear to rotate around the seemingly motionless Polaris (North Star)
Figure 1.2
The stars move across the night sky due to Earth s rotation. Constellations change throughout the year as Earth completes it s revolution around the sun. A star rises 3min. and 56secs earlier each night.
Figure 1.3
The Path the Sun takes across our sky is called the ecliptic because eclipses only occur when the new or full moon cross it. The word zodiac comes from the Greek word for little animal. The Zodiac is a circle or band of animals the 12 constellations.
Figure 1.4
1.2 The Seasons
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Figure 1.6
The Earth s rotational axis is tilted 23.5 degrees from the vertical in relation to its orbit around the Sun. The seasons are caused by the tilt of Earth s rotation axis. In summer, we tilt toward the sun; the light is more intense and the days longer. Seasons are reversed in the Southern Hemisphere.
Figure 1.7
Figure 1.8
The Sun s path, the ecliptic, is tilted with respect to the celestial equator. If you lived at 40 degrees North, on June 21, the Sun would appear at 63.5 degrees (40 + 23.5) above the horizon. On Dec. 21, it would appear at 16.5 degrees (40 23.5) above the horizon.
Figure 1.9
Equinoxes (equal nights) occur when the Sun is on the celestial equator. - Spring Equinox on March 21 - Fall Equinox on September 22 Solstices occur when the Sun (sol) has stopped (stasis) its motion above or below the celestial equator. -Summer Solstice on June 21 - Winter Solstice on December 21
Figure 1.10
From 12/21 6/21, the Sun is a little higher in the sky and rises / sets further north each day. From 6/21 12/21, the Sun is a little lower in the sky and rises / sets a little further south.
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1.3 The Moon
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The Moon rises in the East and sets in the West. The Moon shifts its position across the background of stars from west to east. - Each night it will rise a little later (about 50 min.) The Moon goes through a complete cycle of phases in 29.5 days - The Moon is the origin of the word month.
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A new moon occurs when the Moon lies between the Sun and the Earth. A full moon occurs when the Moon is on the other side of the Earth from the Sun. The Moon s shape changes because as it moves we see different amounts of it illuminated. The Moon goes through 8 phases.
Figure 1.14
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1.4 Eclipses A solar eclipse occurs whenever the Moon passes directly between the Earth and Sun and blocks our view of the Sun (New Moon). A lunar eclipse occurs when the Earth passes between the Moon and and Sun and casts a shadow on the Moon (Full Moon).
Figure 1.16
Some eclipses are partial and some are total. A lunar eclipse appears red because Earth s atmosphere filters out the blue part of the spectrum. Total solar eclipses are much rarer because the Moon s shadow on Earth is quite small.
Figure 1.17
Lunar and solar eclipses are rare because the Moon s orbit is tipped with respect to Earth s orbit. Twice a year alignments create an eclipse season. Eclipses generally occur in pairs. A solar eclipse is generally followed by a lunar eclipse 2 weeks later or vice versa.
Figure 1.19
The Moon s orbit goes through a precession as the plane of orbit slowly shifts. As many as 5 solar and 2 lunar or 4 solar and 3 lunar eclipses can occur a year. At least 2 solar eclipses must occur each year. They are only seen from a small band of Earth.
Table 1.1
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Figure 1.21
Project Figure 1.1