Tegrity Tegrity has been having issues the past few days. Service may be intermittent. Tegrity has been having issues the past few days. Service may be intermittent.

Telescope Viewing Tonight! Professor Buta will be holding an open house at the Gallalee 16” Telescope on the roof (4 th floor) of Gallalee Hall tonight (August 26) from 7:30-9pm. See Saturn and the Moon. Event will be cancelled in the event of clouds, so use your best judgment in assessing the weather situation. Professor Buta will be holding an open house at the Gallalee 16” Telescope on the roof (4 th floor) of Gallalee Hall tonight (August 26) from 7:30-9pm. See Saturn and the Moon. Event will be cancelled in the event of clouds, so use your best judgment in assessing the weather situation.

Homework #1  Due Monday, August 31, 6PM  Designed to get your familiar with the MasteringAstronomy.com interface  Mandatory, but ungraded (as long as you complete it on time, you get full credit). You will receive 100% even if the program says you did not receive a 100%.  Lose 10% per day for every day late, for 5 days – after that you can still complete the assignment until the day before the final exam for 50% credit (this will be true for all homeworks).  Due Monday, August 31, 6PM  Designed to get your familiar with the MasteringAstronomy.com interface  Mandatory, but ungraded (as long as you complete it on time, you get full credit). You will receive 100% even if the program says you did not receive a 100%.  Lose 10% per day for every day late, for 5 days – after that you can still complete the assignment until the day before the final exam for 50% credit (this will be true for all homeworks).

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Chapter 2 Discovering the Universe for Yourself

What can we see in the night sky? With the naked eye, we can see more than 2000 stars as well as the Milky Way, the Large and Small Magellanic Clouds, and the Andromeda Galaxy (barely).

The Milky Way A band of light making a circle around the celestial sphere. What is it? Our view into the plane of our galaxy.

Constellations A constellation is a region of the sky – it has no real astronomical significance. Eighty-eight constellations fill the entire (north + south) sky. Many southern sky constellations have lame names (i.e., Telescopium, Sextans)

We measure the sky using angles. The Sun and Full Moon are ½ degree wide 1 circle = 360 degrees 1 degree = 60 arcminutes 1 arcminute = 60 arcseconds

The Local Sky An object’s altitude (above horizon) and azimuth (along horizon) specify its location in your local sky (specified in degrees). These quantities are dependent on your location on Earth.

The Celestial Sphere North celestial pole is directly above Earth’s North Pole (Polaris is very near here). South celestial pole is directly above Earth’s South Pole. Celestial equator is a projection of Earth’s equator onto sky.

Why do stars (or Sun) rise and set? Earth rotates from west to east, so stars (and Sun, Moon, planets) appear to circle from east (rise) to west (set) in a given day. Altitude of north celestial pole (Polaris) gives your latitude on Earth. North Star (Polaris)

The Celestial Sphere Ecliptic is the Sun’s apparent path through the celestial sphere over the course of a year through 12 signs of the zodiac.

The sky varies as Earth orbits the Sun  As the Earth orbits the Sun, the Sun appears to move slowly eastward along the ecliptic, moving to a new constellation every month.

Movement of the Sun Sun makes a complete circuit in the sky (360 degrees) every 365 days (i.e, from Taurus in 2015 to Taurus in 2016) relative to background stars  Sun moves ~1 degree/day (twice the angular size of the Sun) relative to background stars Sun moves ~30 degrees per month (stays in one zodiac constellation each month) relative to background stars  moves 12 constellations in 12 months (due to Earth’s revolution around the Sun) Of course, Sun (and stars) rise/set together over the course of a single day (due to Earth’s rotation on axis).

Suppose on a given day you wake up at sunrise, and the Sun is in the middle of the constellation Scorpius. In what constellation will it be in at sunset? A) In Scorpius B) One zodiac constellation from Scorpius C) Three zodiac constellations from Scorpius D) Six zodiac constellations from Scorpius A) In Scorpius B) One zodiac constellation from Scorpius C) Three zodiac constellations from Scorpius D) Six zodiac constellations from Scorpius

Suppose on a given day you wake up at sunrise, and the Sun is in the middle of the constellation Scorpius. In what constellation will it be in at sunset? A) In Scorpius B) One zodiac constellation from Scorpius C) Three zodiac constellations from Scorpius D) Six zodiac constellations from Scorpius Remember, the Sun only moves 1 degree per day relative to the background stars, and constellations are ~30 degrees wide. A) In Scorpius B) One zodiac constellation from Scorpius C) Three zodiac constellations from Scorpius D) Six zodiac constellations from Scorpius Remember, the Sun only moves 1 degree per day relative to the background stars, and constellations are ~30 degrees wide.

Obviously wrong, since we have summer when Australia has winter. 23.5° tilt of Earth’s axis is responsible for the different hemispheres receiving different amounts of daily sunlight throughout the year  seasons. For Tuscaloosa: ~14 hours of daylight in summer, ~10 hours in winter For the equator: 12 hours of daylight in summer, 12 hours in winter For North Pole: 24 hours of daylight in summer, 0 hours in winter Astronomy Myth #1 Seasons are caused by the changing distance of the Earth from the Sun during the year.

What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlight. Northern Summer – northern hemisphere points toward Sun Northern Winter– northern hemisphere points away from Sun

Sun’s altitude also changes with seasons. Sun’s position at noon in summer: Higher altitude means more direct sunlight. Sun’s position at noon in winter: Lower altitude means less direct sunlight. Sun image taken about every 2 weeks at noon for a year.

Why doesn’t distance matter? variation of Earth–Sun distance is small-about 3%; this small variation is overwhelmed by the effects of axis tilt. distance does matter for some other celestial bodies -- notably Mars (19%) and Pluto (66%). variation of Earth–Sun distance is small-about 3%; this small variation is overwhelmed by the effects of axis tilt. distance does matter for some other celestial bodies -- notably Mars (19%) and Pluto (66%).

How do we mark the progression of the seasons? We define four special points (Northern hemisphere only!): summer (June 21) solstice – longest daylight winter (December 21) solstice – shortest daylight spring (March 21) equinox - 12 hours daylight fall (September 21) equinox – 12 hours daylight solstice = “sun stops” equinox = equal day and night

We can recognize solstices and equinoxes by Sun’s path across sky: Summer (June) solstice: highest path; rise and set at most extreme north of due east Winter (December) solstice: lowest path; rise and set at most extreme south of due east Equinoxes: Sun rises precisely due east and sets precisely due west. Northern hemisphere only!

How does the orientation of Earth’s axis change with time? Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years.  Polaris won’t always be the North Star in a few thousand years.

What’s Wrong With These Drawings?