Chapter 1

Astronomy Today

7th Edition

Chaisson/McMillan

Grab your text book

Charting the Heavens

Day 3

Sidereal day

• Earth’s true rotation period• Amount of time it takes for our planet to

return to the same spot relative to the stars• Exact amount of time the earth rotate on its

axis• 4 minutes less than a solar day• Solar day: the earth moves forward a small

amount (1 degree) on it’s path around the sun

Earth’s Orbital MotionTwelve constellations (some say thirteen) Sun moves through during the year are called the zodiac; path is ecliptic

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 areIncorrectly 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

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

horizon

star

altitude

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

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