University of Michigan - Department of Astronomy

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Version: Planetarium

The Seasons

I must hear wise talk of the kind of weather, sort of season and time of year

--Robert Browning

Overview

Introduction

Below is a figure showing some of the major guides here on Earth.  We project the Earth’s poles and equators onto the sky to give us the north and south celestial poles and celestial equator.  We also use the ecliptic to help us define the location of the tropics on Earth.  The northern tropic (Cancer) is where an observer would see the sun directly overhead on the summer solstice, and the tropic of Capricorn (at 23.5º S latitude) is where an observer would see the Sun directly overhead on the winter solstice. The tropics were named for the constellation that the Sun was in roughly 4000 years ago when people first started developing these guides and writing them down. See the precession activity for why the Sun is no longer in those constellations.

Major guides on Earth

Below is a figure showing the major lines displayed in the planetarium. Following that are more details on how to read the date, time and position of objects from the planetarium dome.

Celstial sphere

In this activity, you will explore how to use these lines and what causes the different seasons.

If you haven't done the Coordinate Systems activity, you'll need the information below:

Details on determining dates and times:

The local sidereal time (LST) is equal to the right ascension of the meridian. In the planetarium, it can be determined by reading the hour off the equator where it crosses the meridian: e.g. if the second tic after 15 lines up with the meridian, the LST is 15:20. The sidereal day is based on the stars, and is 4 minutes shorter than the solar day. Since you'll need to determine the number of hours of daylight to the nearest half hour, this difference is insignificant.

The date is based on the Sun's position on the ecliptic. In the real sky, the equinoxes and solstices occur on the 21st of March, June, September and December, give or take one day. However due to the play in the planetarium gears the position of the Sun is only accurate to within about 2 days (i.e. the ecliptic crosses the equator on March 21, but the Sun may actually sit on March 19.) Record the position of the Sun when asked for the date. Keep this inaccuracy in mind when doing the lab.

Please note, the wall in the planetarium cuts off the horizon 2º higher than the true horizon.


This first part is meant to make sure you know how to read all the information you need. Your GSI will run the planetairum to the correct positions. MAKE SURE YOUR ALSO RECORD YOUR ANSWERS ON THE WORKSHEET since your GSI may not collect this introduction.

Part 1:Ann Arbor

sunrise

  1. What direction do you have to look to see the Sun rising?



  2. what is the local sidereal time?



Local noon

  1. What is the date?



  2. is the Sun above, below, or on the equator?

  3. is it the vernal equinox, summer solstice, autumnal equinox or winter solstice?  How do you know?







  4. What is the meridian altitude?

Sunset

  1. What direction is the sun setting?



  2. what is the local sidereal time?



  3. Based on the times of Sunrise and Sunset, how many hours was the Sun above the horizon (to the nearest quarter hour)?  Show your work.







Check your answers with your neighbor, or get confirmaiton from your GSI. Make sure you know how to get the answers if you got any wrong, and don't forget to transfer your answers to the worksheet.

Activities:


Last modified: 4/16/08 by SAM with material from EMP.

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