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.
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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.
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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.
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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?