Grab your text book Chapter 1 Astronomy Today 7th Edition Chaisson/McMillan Grab your text book

Charting the Heavens Day 2 High overhead on a clear, dark night, we can see a rich band of stars known as the Milky Way—so-called for its resemblance to a milky band of countless stars. All these stars (and more) are part of a much larger system called the Milky Way Galaxy, of which our star, the Sun, is one member. This single exposure, dubbed “the Going to the Stars Road,” was made at night with only the Moon’s light illuminating the terrain on the continental divide at Logan Pass in Glacier National Park, near the Montana/Alberta border. (© Tyler Nordgren)

Celestial Sphere The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth They aren’t, but they 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.

Celestial Coordinates Right Ascension Declination Like longitude Use units of time-hours instead of degrees 0 hour is the vernal equinox Like latitude except use +/- instead of north and south

Terms related to the Celestial Sphere Terrestrial System Celestial System South Pole North pole Equator Latitude 0° at the Equator Longitude 0° at the Prime Meridian South Celestial Pole North Celestial Pole Celestial Equator Declination 0° at celestial Equator Right Ascension 0 Hours at Vernal Equinox

Angular Measure 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 its actual size and distance from viewer

Earth’s Orbital Motion Daily cycle, noon to noon, is diurnal—solar day Stars aren’t in quite the same place 24 hours later, though, due to Earth’s rotation around Sun; when they are once again in the same place, 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 4 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 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 Twelve constellations (some say thirteen) 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.

Sun signs Are based on which constellation the sun was in on the day of your birth Moon sign: which constellation is the moon is at the time of your birth Most astrological signs are Incorrectly shown based on your birth where the sun was During Greek times.

Ophiucus The thirteenth zodiac sign Sun passes through Ophiucus’ foot November 30-Dec. 18 He is the serpent bearer also used as the medical symbol

Seasons Turn to your elbow partner and discuss why you think we have seasons>WHEN YOU THINK YOU HAVE AN ANSWER < DRAW OR DESCRIBE YOUR ANSWER ON YOUR WHITE BOARD> ALL TABLES WILL BE REPORTING WHAT THEY THINK SO BE PREPARED.

Ecliptic If the sun's path is observed from the Earth's reference frame, it appears to move around the Earth in a path which is tilted with respect to the spin axis at 23.5°. The angle of earth’s axis=the plane of the angle of the ecliptic to the celestial equator

Earth’s Orbital Motion Ecliptic is plane of Earth’s path around Sun; at 23.5° to celestial equator Northernmost point of path (above celestial equator) is summer solstice; southernmost is winter solstice; points where path crosses celestial equator are vernal and autumnal equinoxes Combination of day length and sunlight angle gives seasons Time from one vernal equinox to next is tropical year 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.

Seasons On the Equinox, the Earth experiences equal amounts of day and night Summer solstice- the sun is at it’s highest point Winter Solstice –the sun is at it’s lowest point

Local co-ordinate systems Based on the objects above the plane of the horizon Altitude is the angle above the horizon star altitude horizon NORTH STAR HAS AN ALTITUDE OF 0° IF YOU ARE ON THE EQUATOR THE ALTITUDE OF THE NORTH STAR=YOUR LATITUDE ON EARTH!!!

A ROUGH WAY TO ESTIMATE ALTITUDE PINKY =1° 3 FINGERS=3° FIST = 10°

Local Co-ordinate System Azimuth – starts with north a 0° and south is 180° Zenith is 90° zenith

North Star Even though Polaris is currently the North star, it doesn’t lie due North –and eventually will move Vega will be our North Star, Why do you think this is happening? Discuss with your elbow partner, write down your thoughts on your white board, be ready to defend them.

CIRCUMPOLAR STARS STARS THAT NEVER GO BELOW THE HORIZON CAN ALWAYS BE SEEN AT NIGHT http://upload.wikimedia.org/wikipedia/commons/2/21/Zirkumpolar_ani.gif

The North Star Polaris, our current North Star is the 49th brightest star in the night sky!!!!! To find it, locate the cup of the Big Dipper to the handle of the Little dipper. It doesn’t appear to move in the night sky but the other stars rotate around it North pole and Polaris are about 1° off from one another

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.

Precession Wobbles in a 26,000 year cycle Wobble is between Polaris, Vega and Thuban Changes the position of the Vernal Equinox which will also change the co-ordinates of the stars

SEASONAL CHANGES IN OUR NIGHT SKY Summer triangle: Vega, Denab, Altair Winter: Orion, Sirius (Dog Star) The change occurs about 1°/night

1.4 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