1

PSC101- Introduction to Astronomy

You know Orion always comes up sideways. Throwing a leg up over our fence of mountains,And rising on his hands, he looks in on meBusy outdoors by lantern-light with somethingI should have done by daylight, and indeed,After the ground is frozen, I should have doneBefore it froze, and a gust flings a handfulOf waste leaves at my smoky lantern chimneyTo make fun of my way of doing things,Or else fun of Orion's having caught me.Has a man, I should like to ask, no rightsThese forces are obliged to pay respect to?"So Brad McLaughlin mingled reckless talkOf heavenly stars with hugger-mugger farming,Till having failed at hugger-mugger farming,He burned his house down for the fire insuranceAnd spent the proceeds on a telescopeTo satisfy a life-long curiosityAbout our place among the infinities.

The Star-SplitterRobert Frost

2

What is Astronomy?

• A branch of science. • Studies the motions,

origins, and properties of celestial objects using the scientific method.

• Has its origins in ancient history.

Crescent Jupiter and Io from Cassini Spacecraft

3

What is Astronomy?

• Almost all knowledge is gained through study of light.

• Almost always impossible to perform experiments (too far)

• Astronomy has made great advances with new technologies.

4

Origins of Astronomy - Time• The Sun, Moon and stars served humanity

for thousands of years as a clock and a calendar. Their motions were predictable.

• Today we have other means of measuring time but long ago an accurate understanding of the skies could mean life or death.

• Studying the skies told people when to plant and harvest, when to go out to sea, when to expect good or bad weather, etc.

5

Time - Day & Night

• How do you know what time it is?

• How high is the sun in the sky?

• Morning - sunrise• Evening - sunset• Does the Sun really

rise and set?

Sunrise behind the Space Shuttle

6

Time Day & Night- Earth’s Spin

• In reality because the Earth rotates on its axis from West to East the Sun, Moon and stars all appear to move from East to West.

• The Sun appears to rise in the East and set in the West because the Earth spins.

• The Moon however also orbits the Earth traveling from West to East but it takes much longer than 24 hours to orbit the Earth. However from night to night you can see the Moon change position.

7

Time - The Moon and Months

• It takes the Moon about one month to orbit the Earth

• Our calendar of months was originally based on the Moon

• The word “Month” is derived from the word “Moon”

• Jewish and Islamic calendars are still based on the Moon. The New Moon starts a new month.

Photograph of crescent Moon and crescent Venus

8

Time - Year and Seasons

• Cultures around the world marked the Sun’s yearly motion through the stars.

• Summer - Sun is farthest North.• Winter - Sun is farthest South.• Knowing when these dates

occurred was very important for agricultural societies

First day of Summer at Stonehenge, England

9

Earth’s Orbital Motion• As the Earth

orbits the Sun it appears that the Sun is actually moving.

• The Sun appears to move through a set of 12 constellations over one year.

• Some constellations can only be seen during certain times of year.

10

Earth’s Orbital Motion

The path through the sky along which the Sun appears to move is called the ecliptic

11

Earth’s Orbital Motion

But the planets really do move!As seen from Earth their motionsappear more complicated than theyactually are. In fact they sometimesappear to move backwards! This is called retrograde motion.

The other planets appear to follow similar paths near the ecliptic.The band through which the Sun and the planets appear to move is called the Zodiac