Celestial Coordinates Lab 2

The “Obvious” View Stars that appear close in the sky may not actually be close in space Most prominent stars of Orion Figure 1-9. Caption: Orion in 3-D. The true three-dimensional relationships among the most prominent stars in Orion. The distances were determined by the Hipparcos satellite in the early 1990s. (See Chapter 17.)

The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects Figure 1-11. Caption: Celestial Sphere. Planet Earth sits fixed at the hub of the celestial sphere, which contains all the stars. This is one of the simplest possible models of the universe, but it doesn’t agree with all the facts that astronomers now know about the universe.

The motion of the stars is due to rotation of the earth. The celestial sphere is a fiction. Figure 1-11. Caption: Celestial Sphere. Planet Earth sits fixed at the hub of the celestial sphere, which contains all the stars. This is one of the simplest possible models of the universe, but it doesn’t agree with all the facts that astronomers now know about the universe.

Full circle contains 360° (degrees) Each degree contains 60′ (arc-minutes) Each arc-minute contains 60′′ (arc-seconds) Angular size of an object depends on actual size and distance away

Celestial Coordinates Declination: degrees north or south of celestial equator Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox Equinox:The moment when the Sun is positioned directly over the Earth's equator, and the apparent position of the Sun at that moment (March 21 and September 23 ).

Cosmic lecture launcher Discovering the universe for yourself: Exploring the Motion of the Sky with the Celestial Sphere

Earth’s Orbital Motion Daily cycle, noon to noon, is diurnal motion —solar day Stars aren’t in quite the same place 24 hours later, though, due to Earth’s rotation around Sun; when they are, one sidereal day has passed Figure 1-13. Caption: Solar and Sidereal Days. A sidereal day is Earth’s true rotation period—the time taken for our planet to return to the same orientation in space relative to the distant stars. A solar day is the time from one noon to the next. The difference in length between the two is easily explained once we understand that Earth revolves around the Sun at the same time as it rotates on its axis. Frames (a) and (b) are one sidereal day apart. During that time, Earth rotates exactly once on its axis and also moves a little in its solar orbit—approximately 1°. Consequently, between noon at point A on one day and noon at the same point the next day, Earth actually rotates through about 361° (frame c), and the solar day exceeds the sidereal day by about four minutes. Note that the diagrams are not drawn to scale; the true 1° angle is in reality much smaller than shown here.

Earth’s Orbital Motion Solar day: From noon to noon (sun at the same spot after 24h). Stars at the same spot after one sidereal day(23h56m): Angle covered by earth in 1day: 360 /365d=0.986 /d Earth’s rotation=15 /1h, so it takes 0.986/15*60min=3.9min Figure 1-13. Caption: Solar and Sidereal Days. A sidereal day is Earth’s true rotation period—the time taken for our planet to return to the same orientation in space relative to the distant stars. A solar day is the time from one noon to the next. The difference in length between the two is easily explained once we understand that Earth revolves around the Sun at the same time as it rotates on its axis. Frames (a) and (b) are one sidereal day apart. During that time, Earth rotates exactly once on its axis and also moves a little in its solar orbit—approximately 1°. Consequently, between noon at point A on one day and noon at the same point the next day, Earth actually rotates through about 361° (frame c), and the solar day exceeds the sidereal day by about four minutes. Note that the diagrams are not drawn to scale; the true 1° angle is in reality much smaller than shown here. o o o

Earth’s Orbital Motion Seasonal changes to night sky are due to Earth’s motion around Sun Figure 1-14. Caption: Typical Night Sky. (a) A typical summer sky above the United States. Some prominent stars (labeled in lowercase letters) and constellations (labeled in all capital letters) are shown. (b) A typical winter sky above the United States.

Earth’s Orbital Motion 12 constellations Sun moves through during the year are called the zodiac; path is ecliptic Figure 1-15. Caption: The Zodiac. The view of the night sky changes as Earth moves in its orbit about the Sun. As drawn here, the night side of Earth faces a different set of constellations at different times of the year. The 12 constellations named here make up the astrological zodiac. The arrows indicate the most prominent zodiacal constellations in the night sky at various times of year. For example, in June, when the Sun is “in” Gemini, Sagittarius and Capricornus are visible at night.

Cosmic lecture launcher Discovering the universe for yourself: Sun's Apparent Path through the Zodiac

Earth’s Orbital Motion Ecliptic is plane of Earth’s path around Sun; declined at angle 23.5° to celestial equator Figure 1-17. Caption: Seasons. In reality, the Sun’s apparent motion along the ecliptic is a consequence of Earth’s orbital motion around the Sun. The seasons result from the inclination of our planet’s rotation axis with respect to its orbit plane. The summer solstice corresponds to the point on Earth’s orbit where our planet’s North Pole points most nearly toward the Sun. The opposite is true of the winter solstice. The vernal and autumnal equinoxes correspond to the points in Earth’s orbit where our planet’s axis is perpendicular to the line joining Earth and the Sun. The insets show how rays of sunlight striking the ground at an angle (e.g., during northern winter) are spread over a larger area than rays coming nearly straight down (e.g., during northern summer). As a result, the amount of solar heat delivered to a given area of Earth’s surface is greatest when the Sun is high in the sky.

Earth’s Orbital Motion Northernmost point (above celestial equator) is summer solstice: North pole is closest to the sun. southernmost is winter solstice: North pole is furthest away from the sun. Figure 1-17. Caption: Seasons. In reality, the Sun’s apparent motion along the ecliptic is a consequence of Earth’s orbital motion around the Sun. The seasons result from the inclination of our planet’s rotation axis with respect to its orbit plane. The summer solstice corresponds to the point on Earth’s orbit where our planet’s North Pole points most nearly toward the Sun. The opposite is true of the winter solstice. The vernal and autumnal equinoxes correspond to the points in Earth’s orbit where our planet’s axis is perpendicular to the line joining Earth and the Sun. The insets show how rays of sunlight striking the ground at an angle (e.g., during northern winter) are spread over a larger area than rays coming nearly straight down (e.g., during northern summer). As a result, the amount of solar heat delivered to a given area of Earth’s surface is greatest when the Sun is high in the sky.

Earth’s Orbital Motion Figure 1-17. Caption: Seasons. In reality, the Sun’s apparent motion along the ecliptic is a consequence of Earth’s orbital motion around the Sun. The seasons result from the inclination of our planet’s rotation axis with respect to its orbit plane. The summer solstice corresponds to the point on Earth’s orbit where our planet’s North Pole points most nearly toward the Sun. The opposite is true of the winter solstice. The vernal and autumnal equinoxes correspond to the points in Earth’s orbit where our planet’s axis is perpendicular to the line joining Earth and the Sun. The insets show how rays of sunlight striking the ground at an angle (e.g., during northern winter) are spread over a larger area than rays coming nearly straight down (e.g., during northern summer). As a result, the amount of solar heat delivered to a given area of Earth’s surface is greatest when the Sun is high in the sky. At the equinox, earth axis is perpendicular to the line connecting earth and sun: Day and nights are of equal length.

Earth’s Orbital Motion Time from one vernal equinox to next is tropical year Combination of day length and sunlight angle gives seasons Figure 1-17. Caption: Seasons. In reality, the Sun’s apparent motion along the ecliptic is a consequence of Earth’s orbital motion around the Sun. The seasons result from the inclination of our planet’s rotation axis with respect to its orbit plane. The summer solstice corresponds to the point on Earth’s orbit where our planet’s North Pole points most nearly toward the Sun. The opposite is true of the winter solstice. The vernal and autumnal equinoxes correspond to the points in Earth’s orbit where our planet’s axis is perpendicular to the line joining Earth and the Sun. The insets show how rays of sunlight striking the ground at an angle (e.g., during northern winter) are spread over a larger area than rays coming nearly straight down (e.g., during northern summer). As a result, the amount of solar heat delivered to a given area of Earth’s surface is greatest when the Sun is high in the sky.

Cosmic lecture launcher Discovering the universe for yourself: Why does the flux of the sunlight vary? Flux of light as a Function of Latitude on Earth

Earth’s Orbital Motion Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years Figure 1-19a. Caption: Precession. (a) Earth’s axis currently points nearly toward the star Polaris. About 12,000 years from now—almost halfway through one cycle of precession—Earth’s axis will point toward a star called Vega, which will then be the “North Star.” Five thousand years ago, the North Star was a star named Thuban in the constellation Draco.

Cosmic lecture launcher Discovering the universe for yourself: Precession of Earth's Axis

Earth’s Orbital Motion Time for Earth to orbit once around Sun, relative to fixed stars, is sidereal year Tropical year follows seasons; sidereal year follows constellations—in 13,000 years July and August will still be summer, but Orion will be a summer constellation

Cosmic lecture launcher Discovering the universe for yourself: Much More than Distance - How Earth's Tilted Axis Causes the Seasons

Astronomical Timekeeping Solar noon: when Sun is at its highest point for the day Drawbacks: length of solar day varies during year; noon is different at different locations Figure 1-20. Caption: Variations in the Solar Day. (a) Because Earth does not orbit the Sun at a constant speed, the rate at which the Sun appears to move across the sky, and hence the length of the solar day, also varies. (b) In addition, the length of the solar day changes with the season, because the eastward progress of the Sun in, say, 1 hour (indicated here), is greater at the solstices than at the equinoxes.

Astronomical Timekeeping Define mean (average) solar day—this is what clocks measure. Takes care of length variation. Also define 24 time zones around Earth, with time the same in each one and then jumping an hour to the next. Takes care of noon variation.

Astronomical Timekeeping World time zones Figure 1-21. Caption: Time Zones. The standard time zones were adopted to provide consistent times around the world. Within each time zone, all clocks show the same time, the local solar time of one specific longitude inside the zone.