A100 Solar System The Sun Today Today’s APOD Monday, Sept. 8 Read Chapter 1 in text – History of Astronomy Quiz Today (essential facts and scientific notation) Rooftop Session Sept. 10, 9 PM 1st Homework due Friday (get it on Oncourse) Today’s APOD The Sun Today

First Quiz Bubble and write your name on the Scantron form You may consult with your neighbors No books, notes, papers, internet, cell phones

What can we see in the night sky? With the naked eye, we can see more than 2,000 stars, as well as 5 planets, the Moon, comets, meteors, the Milky Way, and a few other special objects The Milky Way is a band of light that makes a circle around the celestial sphere

Orion rising The night sky is familiar, but few people look closely at (or think about) the complex phenomena we observe in sky.

The Sky! Sky chart for 9 PM TONIGHT North South The Sky! Sky chart for 9 PM TONIGHT The brightness of a star is shown by the size of its dot Jupiter & the Moon in Sagittarius

Jupiter & the Moon in Sagittarius Just look south… Jupiter is the brightest object in the southern sky Sept 8 Sept 9

Check out the just south of west, just after sunset See Mercury, Venus, and Mars You will need to be able to see low on the horizon to the west southwest

Constellations A constellation is a region of the sky Constellation regions are irregular in shape

Do the Stars in a constellation belong together? NO! The stars in a constellation are not all at the same distance from the Sun They do not form a real group in space

The Big Dipper is part of the constellation Ursa Major The stars of the Big Dipper are not all at the same distance from the Sun

The Constellations The 88 official (defined by the International Astronomical Union) constellations cover the celestial sphere

The Celestial Sphere The stars all appear to lie on a large sphere surrounding the Earth (the celestial sphere) Really, the stars are all at different distances

Angular Measurements Full circle = 360º 1º = 60 (arcminutes) 1 = 60 (arcseconds)

We measure the sky using angles

Measure the Angular Size Image taken by the Galileo spacecraft (Jupiter, 1995-97) during its lunar fly-by in December, 1992 The dark areas are lava rock filled impact basins (mare) Tycho impact basin The “real” Moon is ½ degree across

Angular Size distance angular size physical size An object’s angular size appears smaller if it is farther away The angular size of Mars changes depending on where Earth and Mars are in their orbits

The Local Sky An object’s altitude (above horizon) and direction (along horizon) gives its location in your local sky

Definitions: The Local Sky Zenith: The point directly overhead Horizon: All points 90° away from zenith Meridian: Line passing through zenith and connecting N and S points on horizon

Our view from Earth: A circumpolar star never sets Celestial equator Stars near the north celestial pole are circumpolar and never set. All other stars (and Sun, Moon, planets) rise in east and set in west. A circumpolar star never sets Celestial equator This star never rises in Indiana

Why do stars rise and set? Stars in the northern sky circle the northern celestial pole (in the southern hemisphere stars circle the southern celestial pole)

The Earth spins under Polaris (Polaris is NOT the brightest star in the sky!)

North South Big Dipper’s on its side in the northwestern sky Finding Polaris Find an open area away from nearby lights where you have a good view of the sky Find the Big Dipper in the northwestern sky Sky chart for 9 PM TONIGHT

Finding Polaris To find Polaris: find the Big Dipper follow to Polaris the Big Dipper is ALWAYS up in Indiana! Finding Polaris

Altitude of the celestial pole = your latitude

Why do the constellations we see depend on latitude and time of year? They depend on latitude because your position on Earth determines which constellations remain below the horizon. They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars.

The sky changes as Earth orbits the Sun As the Earth orbits the Sun, different constellations are visible at night At midnight, the stars on our meridian are opposite the Sun in the sky

Where on the Earth is it possible to see the most stars through the year? a) From the north pole b) From latitude 23.5 degrees c) From the equator d) anywhere, since latitude makes no difference.

Precession! Over time, the direction of the Earth’s rotation axis drifts around a circle. At different times, different stars (other than Polaris) are close to the pole The location of the celestial pole traces out a circle in the sky every 26,000 years

Vocabulary Review Constellation Ecliptic Equator Celestial sphere Latitude Longitude Meridian Zenith Horizon Altitude North and south celestial poles

One question I’ve always had about astronomy is…

Dates to Remember ASSIGNMENTS this week Read Chapter 1, History of… Kirkwood Obs. open Weds. Rooftop Sky Viewing Weds. 1st HW due Friday