ASTRONOMY CHAPTER 2: THE NIGHT SKY

THE NIGHT SKY IS THE REST OF THE UNIVERSE AS SEEN FROM OUR PLANET. Beyond our atmosphere is empty space. Our planet rotates on its axis once a day.

The sky appears to revolve around the Earth. The sun and stars rise in the East and set in the West.

THE STARS CONSTELLATIONS Orion the Hunter – a hero Constellations – groups of stars that have been named to celebrate heroes, gods,

THE STARS CONSTELLATIONS And mythical beasts.

Names originated in Mesopotamia 5,000 years ago. Additions by Babylonian, Egyptian, and Greek astronomers.

Look at the map and you see all of these cultures gathered around the Mediterranean Sea. 48 of these names are still used today.

Later, the Roman Empire included most of these areas, the ancient astronomers traveled and shared information.

Today 88 official constellations that each represents an area of the sky. Names come from Greek translated into Latin

Asterisms – less formally defined groupings Example: Big Dipper (part of Ursa Major) and Great Square (in Pegasus) In France it is the “saucepan”, in Britain, it is the “Plow”, the American Indians saw a bear with hunters tracking it, runaway slaves called it the “drinking gourd” and followed it north to freedom.

The pointer stars in the Big Dipper point the way to Polaris, the end of the Little Dipper, which is Polaris

The stars in the constellation may not actually be close to each other, they just lie in the same direction from Earth.

THE NAMES OF THE STARS The brighter stars have individual names. Most star names come from ancient Arabic. – Betelgeuse (shoulder of Orion) – Sirius (the scorched one) – Aldebaran (follower of Pleiades)

Greek letters are assigned to the bright stars in a constellation in approximate order of brightness. The brightest star would be alpha (  )

To identify a star you give the Greek letter followed by the possessive form of the constellation name. Ex:  Canis Majoris is the brightest star in the constellation Canis Major β Canis Majoris would be the second brightest and so on.

BRIGHTNESS OF STARS PTOLEMY Hipparchus in 150 B.C. and Ptolemy about 140 A.D. both came up with magnitude systems

Magnitude scale – measures the brightness of stars. – Brightest are negative magnitudes – Very Bright are 1 st magnitude – Dimmest are 6 th magnitude (faintest visible to the human eye)

Apparent visual magnitudes (m v )– describe how the stars look to human eyes from Earth.

Flux –a measure of the light energy from a star that hits one square meter in one second. Defines the intensity of starlight and is more accurate.

THE SKY AND ITS MOTION THE CELESTIAL SPHERE (A SCIENTIFIC MODEL) Ancient astronomers believed the sky was a great sphere surrounding Earth with the stars stuck on it. We still use it today to find objects in the sky easily. The eastward rotation of Earth causes sky objects to appear to move westward in the sky.

We see the Northern half of the celestial sphere above the horizon since we live in the Northern hemisphere.

Zenith – the top of the sky above your head Nadir – the bottom under your feet

North Celestial Pole – above the Earth’s North Pole. The North Star, Polaris is the pivot point. It stays in place and the stars seem to rotate around it. South Celestial Pole - above the South Pole.

Star Trails created by leaving the camera shutter open show that the stars do revolve around Polaris, the North Star.

Circumpolar constellations – circle Polaris and are always in the night sky (never set)

Celestial Equator – halfway between the North and South poles is a plane that extends outward from Earth and intersects the celestial sphere.

North point and South point are the points on the horizon closest to the celestial poles. East point and West point lie halfway between the north and south points. The celestial equator always meets the horizon at the east and west points.

EQUATORIAL SYSTEM Declination – (like latitude) the angular distance north or south of the celestial equator. angular distance – measures the distances on the celestial sphere in degrees. Measured in degrees N+ or S- of the celestial equator. 60’ minutes of arc per degree, and 60” seconds of arc per minute.

Right ascension – (like longitude) the angular distance measured eastward along the celestial equator from the position of the vernal equinox. Measured in 24 hours. One hour is divided into 15 0 degrees, which can be further divided into secs.

The sun and moon are about ½ a degree in diameter. The bowl of the Big Dipper is about 10 0 wide

Precession – a slow motion of Earth’s axis which traces out a circle in the sky over a period of 26,000 years. Caused by Earths large mass, rapid rotation, and the gravity of the sun and moon. Now our axis points to Polaris at from vertical

14,000 A.D. it will point to Vega which will become our North Star. Earth’s tilt changes only slightly.