University of Michigan - Department of Astronomy

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Version: Starry Night

The Seasons

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

--Robert Browning

Overview

Introduction

Major guides on EarthFigure 1: The major latitude lines 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 (see fig. 2 below.)  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.

 

Figure 2: Lines on the celestial sphere as they appear in the planetarium (line are similar on the celestial spheres and in Starry Night)celestial sphere

Starry Night can replicate the sky from anywhere on Earth at nearly any time. It also allows you to put some guide lines up so that you can make accurate measurements of positions.

In the first part of this activity, you'll observe the position and motion of the Sun on several days from a couple different locations on the Earth. Based on your observations, you'll be able to answer some questions, and develop an understanding of the true cause of the seasons.

If you have never used Starry Night before, you may want to download the Starry Night Quick Reference (pdf). You may also want to view the hints for the lab computers (pdf).


Procedure

Open Starry Night. It should automatically open to Now in Ann Arbor. The Home button should return you to this if you need to come back. You can also quit (without saving) and re-open to get back to this state. If you save, you will need to reboot the computer to get back to the default settings. There may be "Favotites" under the Favotites tab on the left for this activity. If not, follow the steps below (you may want to look the steps over anyway.)local horizon controlstop time flow

Stop the time flow (see image to left)

In the Options tab, in the Local View area, click "Local Horizon" (see image to right). Select "flat horizon" from the dialogue box. You may want to turn Daylight off as well.

Which guides should be checkedIn the Options tab, Guides section:

under Alt-Az check Meridian;

under Celestial check Equator, Summer/Winter Solstice and Vernal/Autumnal Equinox;

and under Ecliptic check Equator. (See image to left)

pop up menu with centre command

Go to the Find tab and check the box next to "Sun". Click the arrow next to the check box (see image to right)and select Centre from the pop-up menu. If it tells you the Sun is not currently visible from your location, select "Best Time" from the dialogue window.

 

Appearance of the Sun on the equinoxAdjust the date and time until the Sun is centered on the Autumnal equinox (hint: start by setting the date to September 21 of this year).

  1. What is the date, including the year?

Sunrise

Click the Sunrise button. The display should automatically change to sunset, with the Sun near the middle of your screen.

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

  2. What is the local time?

Local noon

In the info tab under Sun Info look up the time of the Sun's transit (if the top doesn't say Sun Info, go back to the find tab and make sure the Sun is the only box checked). Run the time flow forward by minutes to that time (you can click the button, but it may be helpful to actually watch the Sun rise, transit, and set). The Sun should be on the meridian line.

  1. is the Sun above, below, or on the celestial equator (circle one)?

  2. This point is labeled the Autumnal Equinox. If it wasn't labeled, how could you tell this was (a) an equinox and (b) specifically the autumnal one?


  3. What is the meridian altitude?

Sunset

Click the Sunset button to change to sunset on this day.

  1. What direction is the sun setting?

  2. What is the local 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 another group, or get confirmation from your GSI. Use the information above to fill in the first row of table 1. You'll fill out the rest of the table using the same basic procedure.

The next row is labeled Winter Solstice. Change the date to Dec 21, then adjust the date to get the Sun to sit on the Winter Solstice mark. Repeat the above steps to go to sunrise, transit, and sunset to get the information you need for that row of the table.

Repeat this again for the other 2 rows, starting from March 21 for the vernal equinox, and June 21 for the summer solstice.

To fill out the remaining table(s) on your worksheet, you will need to change your location. The top of each table tells you which location to go to, depending on which version you are doing. Click the arrow on the right side of the Viewing Location toolbar and select "other" from the drop down menu. Select the Latitude/Longitude tab. Leave the Longitude as it is (83° 44.391' W) for all locations. Change the latitude to the following numbers for the other locations:

Chile: 42° 17.093' S (change the N to S if starting from Ann Arbor.)

Equator: 0° (delete the whole field and enter a 0. You do not need to keep the degree or minute marks or the N or S)

North Pole: 90° (delete the whole field and enter a 90. You do not need to keep the degree or minute marks or the N)

Set the date to the same date as the first row of table 1 (watch the year!). The dates in all your tables should be the same.

For Chile or the Equator, follow the same procedure as above in order to fill out the tables.

At the North Pole, if you turned daylight off, turn it back on. Set the time to 12:01 AM (make sure the date is still the same). Set the time flow rate to 5 minutes and let it run forward and watch what the Sun does. After you've gone around a full day, back it up until the Sun is over the S mark on the horizon, then fill out the first row of the table. To go to the next date, change the time flow to 1 days, run forward and watch what happens. Stop when you get to the correct date, or use the Time and Date toolbar to fine-ute the date. Set the time to 12:01 again and the time flow to 5 minutes, then run forward and watch what happens. If the Sun never comes above the horizon, you can get the altitude from the Info tab.

Once you have all the tables filled in, answer the questions. You should be able to answer all the questions using the data in the tables, but you may find the computer useful for some of them.

Activities:


Last modified: 4/24/08 by SAM.

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