Astronomy 101: 9/18/2008 Announcements Pick up a golf ball at the front of the class or get one from Alex; you will need it for an in-class activity today. You will also need the question sheet from Alex. My office hours will be on Friday from 1-3 PM this week The first real OWL homework is now available. Called Homework 1 Due 9/24 before midnight An astronomer specifies an object s location using two angles. This method of specifying an address is familiar: Coordinates on the Earth Latitude: angle to position north or south of equator Longitude: angle to position east or west of prime meridian (runs through Greenwich, England) Orchard Hill Observatory Open every Thursday evening if skies clear! Located at the base of the cell phone tower, go from Orchard Hill Dorms following the gravel road to the left. Call 577-4166 after 6pm on Thursdays to check whether the telescope will be open. Show your ID and tell the TA that you are in this section of Astro 101. Bring a flashlight and go with friends the path is dark! Dress warmly! Observing the sky: The Earth Gets in the Way... We can t see below the horizon (we can t see through the Earth!). So, we need to have telescopes in different locations, and we have to think about the timing of the Earth s rotation when planning observations. The Earth is constantly rotating, so a telescope has to constantly move to follow a star or galaxy 1
The Celestial Sphere -- a model for mapping the sky For mapping, we treat the stars as if they are all at the same distance on the surface of an imaginary sphere This is a quite useful model, but it is an awful theory PRS Question: Most stars rise and set, but some never rise or set (the circumpolar stars). Is this statement universally true for any observer on Earth? 1. Yes. At any place, some stars will be circumpolar, and some will rise/set. 2. No. Consider an observer at the North or South Pole: At midnight, the star is here. The observer can still easily see the star. At noon, a star is here. The observer can easily see it at noon. The way stars move also differs from place to place Rotate into local orientation: At the North or South Pole, all stars are circumpolar. Nothing rises or sets!! Stars don t necessarily rise straight up from the horizon. A telescope has to know the angle at which objects rise so that it can follow the star as time passes. 2
The way stars move also differs from place to place Rotate into local orientation: Rotate into local orientation: Non-circumpolar stars, (& Sun, Moon, planets) rise in East and set in West at an angle = [90 your latitude]. Hey! Hey, wait a sec This angle is the observer s latitude on the Earth. This angle is the altitude of the star Polaris (which is directly above the north celestial pole). Using geometry, we can show that these angles are the same! Navigation by the stars The altitude of Polaris above the horizon is approximately the same as the observer's latitude in the Northern Hemisphere. Hey! By measuring the altitude of Polaris, one can determine one s latitude on the surface of the Earth. 3
The Sun gets in the way too? Stars don t disappear in the daytime. Where would they go? Stars can t be seen with the naked eye during the day because they are lost in the glare of light from the Sun. Annual Motion Seasons are due to the tilt of the earth s axis. As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. The Sun circles the celestial sphere once every year. The path that the Sun follows through the celestial sphere is the ecliptic. The constellations along the ecliptic are the zodiac. The Sun is just another star, and we can think about its daily motion through the sky the same way. 4
Seasons are due to the tilt Imagine of the this earth s axis of axis. rotation instead. PRS QUESTION: Suppose that the tilt of the Earth s axis of rotation were 90 degrees instead of 23.5 degrees, and the North Pole faces the Sun in June. HOW MANY HOURS OF SUNLIGHT WOULD WE HAVE IN AMHERST IN DECEMBER? Day length = 24 hours Day length = 0 hours Day length = 12 hours In this imagined Solar System, the amount of daylight in Amherst at different times of year would be as indicated The land of midnight Sun Position of the Sun over the course of a day north of the Arctic Circle in the summertime At this location and time, the Sun is circumpolar. Consider the angle between the zenith and the Sun at noon: In the summer, the angle between the Sun and the zenith is small Midnight 6:00 AM Noon 6:00 PM In the winter, the angle between the Sun and the zenith is much larger. So what? 5
Consider the angle between the zenith and the Sun at noon: Going from winter into summer, the Sun rises to higher altitudes and stays in the sky longer Reasons the Earth s tilt causes the seasons The days are longer in the summer; the Sun spends more time heating things up. Sunlight is more direct and more concentrated in the summer. Beginning of the seasons: Summer solstice: the North Pole is tilted toward the Sun; longest day of the year in the Northern Hemisphere (shortest in the south) Winter solstice: the North Pole is tilted away from the Sun Spring and Fall equinox: sunlight shines equally on both the Northern and Southern Hemispheres. Seasonal Change in Sun s Altitude The Figure 8 shows Sun at same time each day over a year. 6
To pinpoint an object in the local sky, we simply specify two angles: altitude the angle above the horizon azimuth An angle between a defined point (say, due West) and the place on the horizon from which we measure the altitude. Problem: the local sky depends on time and place. More often than not, astronomers measure azimuth with respect to due North, but it doesn t matter the reference point is arbitrary as long as it is specified. Astronomers are all over the world and measure coordinates at all sorts of times. In order to talk to each other, astronomers measure coordinates with respect to defined reference points on the celestial sphere, not the local sky. Declination: angle from the celestial equator up to the star (analogous to altitude) Spring (or vernal ) equinox: point where the ecliptic and celestial equator intercept Right Ascension: east-west angle between the spring equinox and the position of the star on the celestial equator. Celestial motions define the cycles of our lives Daily/diurnal motions due to spin of the Earth: day Annual motions due to the orbit of the Earth around the Sun: year What defines the month? Answer: The Moon Phases of the Moon s 29.5 day cycle Why does the Moon s appearance change this way over the course of the month? Moon Moonth Month 7