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2.1 The sky above  (Page 4/17)

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Rising and setting of the sun

We described the movement of stars in the night sky, but what about during the daytime? The stars continue to circle during the day, but the brilliance of the Sun makes them difficult to see. (The Moon can often be seen in the daylight, however.) On any given day, we can think of the Sun as being located at some position on the hypothetical celestial sphere. When the Sun rises—that is, when the rotation of Earth carries the Sun above the horizon—sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars above the horizon.

For thousands of years, astronomers have been aware that the Sun does more than just rise and set. It changes position gradually on the celestial sphere, moving each day about 1° to the east relative to the stars. Very reasonably, the ancients thought this meant the Sun was slowly moving around Earth, taking a period of time we call 1 year    to make a full circle. Today, of course, we know it is Earth that is going around the Sun, but the effect is the same: the Sun’s position in our sky changes day to day. We have a similar experience when we walk around a campfire at night; we see the flames appear in front of each person seated about the fire in turn.

The path the Sun appears to take around the celestial sphere each year is called the ecliptic    ( [link] ). Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars. Earth must make just a bit more than one complete rotation (with respect to the stars) to bring the Sun up again.

Star circles at different latitudes.

The Apparent Motion of Stars at Different Latitudes. Panel (a) depicts the Earth with the North and South Poles vertically aligned. The Earth is embedded in a sphere representing the sky. An observer is drawn standing on the North Pole. Both the zenith and North celestial pole are labeled on the sky directly above the observer. The horizon of this observer, drawn in red, is also projected onto the sky. White circular arrows are dawn counter-clockwise around the zenith/North celestial pole indicating the apparent motion of stars from the observer’s vantage point. In this case stars circle the North celestial pole and never set below the horizon. Panel (b) depicts the Earth with the North and South Poles horizontally aligned. The Earth is embedded in a sphere representing the sky. An observer is drawn standing on the Equator. The zenith is labeled on the sky directly above the observer. The horizon of this observer, drawn in red, is projected onto the sky. The North celestial pole is labeled and lies on the observer’s horizon. White circular arrows are dawn counter-clockwise around the North celestial pole indicating the apparent motion of stars from the observer’s vantage point. In this case all stars rise in the East and set in the West. Panel (c) depicts the Earth with the North and South Poles aligned at a 45-degree angle from horizontal. The Earth is embedded in a sphere representing the sky. An observer is drawn standing in the Northern Hemisphere. The zenith is labeled on the sky directly above the observer. The horizon of this observer, drawn in red, is projected onto the sky. White circular arrows are dawn counter-clockwise around the North celestial pole indicating the apparent motion of stars from the observer’s vantage point. In this case stars close to the celestial poles do not set, those farther from the celestial poles rise in the East and set in the West.
The turning of the sky looks different depending on your latitude on Earth. (a) At the North Pole, the stars circle the zenith and do not rise and set. (b) At the equator, the celestial poles are on the horizon, and the stars rise straight up and set straight down. (c) At intermediate latitudes, the north celestial pole is at some position between overhead and the horizon. Its angle above the horizon turns out to be equal to the observer’s latitude. Stars rise and set at an angle to the horizon.

As the months go by and we look at the Sun from different places in our orbit, we see it projected against different places in our orbit, and thus against different stars in the background ( [link] and [link] )—or we would, at least, if we could see the stars in the daytime. In practice, we must deduce which stars lie behind and beyond the Sun by observing the stars visible in the opposite direction at night. After a year, when Earth has completed one trip around the Sun, the Sun will appear to have completed one circuit of the sky along the ecliptic.

Constellations on the ecliptic.

Constellations on the Ecliptic. The Sun is drawn at the center of this figure. Surrounding the Sun is a blue circular arrow indicating the path of the Earth around the Sun. The Earth is drawn in two positions along this arrow, representing where is it located in June and August. Surrounding the circle of the Earth the constellations of the ecliptic are drawn. Moving counter-clockwise from top center are: Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricorn, Aquarius, Pisces, Aries, Taurus, and back to Gemini. As the Earth moves around the Sun throughout the year, our vantage point changes. This is illustrated with an arrow drawn from the Earth through the center of the Sun to the constellation behind the Sun as seen from Earth. In June the arrow points to Taurus, meaning that the Sun is “in” Taurus in June and is not visible in the night sky. In August the arrow points to Cancer, meaning that the Sun is “in” Cancer in June and is not visible in the night sky.
As Earth revolves around the Sun, we sit on “platform Earth” and see the Sun moving around the sky. The circle in the sky that the Sun appears to make around us in the course of a year is called the ecliptic . This circle (like all circles in the sky) goes through a set of constellation s. The ancients thought these constellations, which the Sun (and the Moon and planets) visited, must be special and incorporated them into their system of astrology. Note that at any given time of the year, some of the constellations crossed by the ecliptic are visible in the night sky; others are in the day sky and are thus hidden by the brilliance of the Sun.
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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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