The Solstices and Equinoxes

        Since the Earth is tilted relative to the plane defined by the Earth’s orbit around the Sun, the apparent path of the Sun is not along the Celestial Equator, but instead follows a path in the sky known as the “ecliptic”. The ecliptic is tilted by 23.5 degrees relative to the Celestial Equator. Over the course of one year, the Sun completes one cycle around the ecliptic. On the following four dates, the Sun is at the following special locations: Mar. 21: the Vernal Equinox the first day of spring (the Sun crosses the Celestial Equator, moving north) June 21: the Summer Solstice the first day of summer (the Sun is as far north on the Celestial Sphere as it gets) Sept. 21: the Autumnal Equinox the first day of autumn (the Sun crosses the Celestial Equator, moving south) Dec. 21: the Winter Solstice the first day of winter (the Sun is as far south on the Celestial Sphere as it gets) In this exercise we will use Stellarium to explore three of the effects of the Earth’s tilt by checking how the Sun’s behavior compares on these various dates. These behaviors are related to the seasons. In particular, we will measure: (1) the length of the day; (2) the compass positions where the Sun rises and sets; and (3) the altitude of the Sun at noon. 1) Set the location. We are all going to use San Francisco as our location so that we can all see the same constellations. At the bottom of the webpage you should see a button that tells you where you’re observing from. It’ll say “near (location),” click that button. Once the map pops up, drag the location pin to San Francisco and click “> use this location” above the map. Also make sure that the toggle for “Use Autolocation” is turned off. (c) 2020 SFSU. DO NOT DISTRIBUTE 11 2) Turn on/off icon features. At the bottom, you’ll also see a bunch of symbols which will turn on and off certain features of the night sky. Turn on the “Constellations” and turn off the “Atmosphere” symbol as shown below. If this isn’t done, you won’t be able to see the stars! 3) Turn on the Meridian. Click the three horizontal line icon in the top left of the screen and look for “View Settings”. Open the settings and check the box that says “Meridian Line”. Once done, you can close out of the settings menu. If you look around the sky, you should notice that the Meridian is a line in the sky (much like the Prime Meridian on Earth) that goes from due North on the horizon through your Zenith (90`aaltitude) to due South again on the horizon. It essentially divides the sky into an Eastern and Western halves. (c) 2020 SFSU. DO NOT DISTRIBUTE 12 5. Make sure to turn South for this exercise. Fill in the tables below. For each position, use Stellarium to observe and record the following things about the path of the Sun during the course of the day. Use the Altitude coordinate grid scale to determine the altitude at noon. Use the time menu (bottom right) to change the time of day, the “hour” of sunrise and sunset and thus the total number of daylight hours. Sunrise is when the Sun is on the Eastern horizon, noon is when the Sun is on the meridian and sunset is when the Sun is on the Western horizon. (c) 2020 SFSU. DO NOT DISTRIBUTE 13 6. At what time(s) of the year is the length of the day equal to the length of the night? 7. How many more hours of daylight are there on the summer solstice than on the winter solstice? 8. Would you expect the temperature to be warmer or cooler than the yearly average when the hours of daylight are longest? 9. How much higher in the sky is the Sun at noon on the Summer Solstice than on the Winter Solstice?