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 STARSCONSTELLATIONS

• Orion the Hunter – a hero

• Constellations – groups of stars that have been named to celebrate heroes, gods,

THE STARSCONSTELLATIONS

• 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 1st magnitude–Dimmest are 6th magnitude (faintest

visible to the human eye)

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• Apparent visual magnitudes (mv)– 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 MOTIONTHE 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 00-900

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 150

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 100 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 23.50 from vertical

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

changes only slightly.