Constellations Constellations -- groups of stars named by ancient cultures to honor gods, animals, legends, etc. They provide us with ways to recognize and identify individual stars and groups of stars (i.e., they are a crude “map” of the sky). Like states

How Many Constellations? In all, there are 88 constellations; 12 in the zodiac (13 really). To learn more about constellations see: The Stars by H.A. Rey http://StarrySkies.com/ Sky & Telescope and Astronomy magazines

Celestial Sphere 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

Eastern Sky in the Winter Orion is a primary Winter constellation

Eastern Sky in the Summer The Summer Triangle

Constellations, cont Stars that appear close in the sky may not actually be close in space:

Big/Little Dippers From our latitude on the Earth, Polaris (the North Star) and the stars around it can be seen all night long. Describe the Big Dipper as the “home base” for finding all of the other constellations. The stars appear to rotate around Polaris.

Big Dipper high overhead. MW on horizon Double, Mizar and Alcor, Mizar itself is a double. Arc to Arcturus in the constellation Bootes. Like an ice cream cone. Corona Borealis, kid licking the ice cream. Spike to Spica in Virgo. Many galxies near here Follow arc in the other direction to get to Leo.

First star to appear after sunset is Vega First star to appear after sunset is Vega. One member of the summer triangle. 2nd star is Deneb, the tail of Cynus the swan. Albireo, at the head of the swan is a famous binary star. Largest color contrast of any binary. 3rd in triangle is Altair in Aquila, the Eagle. At the southern horizon is Sagittarius, the teapot and the direction of the center of the Galaxy. Also, Scorpius, with orange Antares, a red supergiant that means “Rival of Mars”.

Big Dipper is low in the North Cassiopea, the Queen is high overhead. W or M shaped Great Square of Pegasus is to the South. Extend line from left side south, to get to Folmalhaut in Piscis Austrinus. West of Taurus is the Pleides star cluster (M45), seven sisters.

Orion, red supergiant Betelguese, Blue supergiant Rigel Draw a line between the two to find Gemini, the Twins, Castor and Pollux the brightest Follow the belt to the left to Sirius, the Dog Star (brightest star we can see), in Canis Major To the right to get to Taurus, the bull and Aldebaran, which is orangish Above Orion, find Auriga and yellow Capella, its brightest star. Don’t forget the Orion Nebula, a star forming region just below Orion’s belt.

Constellation Boundaries Asterism is the name of the picture. Constellation is like the states. Polaris/Philly famous enough, don’t need state But epsilon blah and Fredericksburg, you do. Names of stars. Buying star names. A constellation is not just the stars that make up a “picture”, it is an area of sky that borders the surrounding constellations.

Celestial Sphere 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

Right Ascension and Declination Declination: degrees north or south of celestial equator Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

The Celestial Sphere Equator => Celestial Equator North Pole => North Celestial Pole South Pole => South Celestial Pole Celestial Sphere is imaginary, not real. It is used for convenience.

What happens when you need to know where an object is more accurately than “the fourth brightest star in Taurus”? Define a new coordinate system: the Celestial Sphere The celestial sphere is essentially a projection of Earth’s coordinate system of longitude and latitude out into space. longitude Right Ascension (RA) latitude Declination (Dec)

So, instead of saying “The object is just to the left of Rigel in the constellation of Orion”, we have coordinates much like longitude and latitude on the Earth.

SkyGazer Demo talk about why astronomers don’t know the constellations.

Rotation of the Earth Earth’s rotation around its axis: Causes night and day (Solar day = average time between consecutive “noontimes”). Causes (apparent) motion of the stars

Years 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 Sidereal year is slightly longer than a tropical year due to precession.

Daily/Nightly Motion of the Stars The rotation of the Earth causes the stars in the night sky to rotate. Depending on where you are on the Earth, the pattern of rotation is different. Stars and constellations that never set (as seen from your location) are called circumpolar (they rotate around the pole).

Circumpolar stars rotate around Polaris (in the Northern Hemisphere).

At the North and South Pole, the stars move horizontally.

On the Equator, the stars move vertically.

At mid-latitudes, the stars move “diagonally” on an arc across the sky.

Daily Motion Position tutorial page 1-2

Yearly Motion of the Stars So far we have just talked about the motion of the stars over the course of a night (due to the Earth’s rotation). Over a day, the Sun will not appear to move with respect to the constellations. But over the course of months or a year, the Sun appears to move.

Yearly Motion of the Stars 12 constellations Sun moves through during the year are called the zodiac; path is the ecliptic

Yearly Motion Ecliptic tutorial page 13-17 (I-III, IV opt) Seasonal Stars tutorial page 7-9 (take home if don’t finish), maybe do this after Horoscopes depending on time. (Fall 2008 – completely ran out of time!)

Today’s Horoscope click here

Horoscopes Your sign The previous sign The next sign Six signs later Read your horoscopes. Which one sounds most like you? Your sign The previous sign The next sign Six signs later Read, take poll of which one is closest. Poll the class and get some data. Open a sky chart showing the location of the Sun on that date. Does it agree with their sign? Avoid dates around the 18th (if I recall correctly) where it is possible for the sign to still be right. Your “sign” is the zodiacal constellation that the Sun was in on the day you were born. But these were defined 2000 years ago. Due to precession, the constellation that the Sun is in when you were born is now shifted by ~1. So, if you want to believe in horoscopes, You should at least read the right one!

Precession – Why Your Sign is Wrong Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years talk about this next time because it relates to the seasons?

Years: Two Types Time for Earth to orbit once around Sun, relative to fixed stars, is a sidereal year A tropical year is the time it takes for the Earth to be in the same configuration with respect to the Sun again (as opposed to distant stars). Tropical year follows seasons; sidereal year follows constellations – in 13,000 years July and August will still be summer, but the “Summer Triangle” will be a winter constellation. Sidereal year is slightly longer than a tropical year due to precession.

Next Time Seasons and Lunar Phases Reading: E2, E3 5.2 (12 pages) HW #1 Due