Questions:
How does the Sun Move?

Introduction

In this brief activity you will explore the way the Sun moves in the sky at different times of year for observers in different locations on the Earth. You will do so using the simulators available at the Nebraska Astronomy Applet Project site on the Motions of the Sun page (http://astro.unl.edu/naap/motion3/motion3.html). To do so you must become familiar with a few simple concepts: azimuth, transit, the ecliptic, and the zodiac.

Azimuth

Azimuth is a way to specify the location of an object in the sky. It is related to Altitude, which you have already learned about. Altitude is how high an object is from the horizon in degrees. For example, the Zenith is 90° from every point on the horizon. Azimuth is the point on the horizon that you use as the starting point to measure Altitude. It is measured in degrees around the horizon starting with 0° at due North (True North, the direction toward the Earth’s rotational axis). It then moves around the circle of the horizon as follows: NNE (22.5°), NE (45°), ENE (67.5°), E (90°), ESE (112.5°), SE (135°), SSE (157.5°), S (180°), SSW (202.5°), SW (225°), WSW (247.5°), W (270°), WNW (292.5°), NW (315°), NNW (337.5°).

Azimuth is used to give the coordinates of an object in the sky (along with Altitude). Just like Altitude it depends on the observer’s location. Azimuth is also useful for defining the point on the horizon where the Sun rises and sets. This is how you will be using Azimuth in this activity.

Transit

To transit, in astronomy, is to cross the oberserver’s meridian. Remember, the meridian is the line the goes from due North (Az. 0°) to due South (Az. 180°) through the Zenith. For example, the traditional definition of noon is the moment that the Sun transits from the point of view of the observer. That is, the moment that the Sun crosses the meridian is called noon. The altitude of the Sun at the moment of transit is an important factor in determining the cause of the seasons on Earth.

Ecliptic

The Ecliptic is the path of the Sun through the sky as seen from the point of view of observers on Earth. Technically, the Ecliptic is the projection of the Earth’s orbit of the Sun on the sky.

In the Northern Hemisphere the beginning of each season can be thought of as follows. On the Vernal Equinox (the first day of Spring) the Sun crosses the Celestial Equator heading North. On the Summer Solstice (the first day of Summer) the Sun reaches its northernmost point, 23.5° north of the Celestial Equator. On the Autumnal Equinox (the first day of Autumn) the Sun crosses the Celestial Equator again on its way South. Finally, on the Winter Solstice (the first day of Winter) the Sun reaches its southernmost point, 23.5° south of the Celestial Equator.

Zodiac

The Zodiac is the group of constellations through which the Sun travels along the Ecliptic. These are the familiar constellations referred to by astrologers. Astrologers are a bit behind the times with regard to how they assign you to the various Sun Signs. When you are born the Sun can be found in one of the signs of the Zodiac: that is, the Sun appears, from our point of view on Earth, to have one of the constellations of the Zodiac behind it. That is your sign. The astrologers give the following dates for each sign:

Sun Sign Dates
* Aries - March 21 - April 20
* Taurus - April 21 - May 21
* Gemini - May 22 - June 21
* Cancer - June 22 - July 22
* Leo - July 23 -August 21
* Virgo - August 22 - September 23
* Libra - September 24 - October 23
* Scorpio - October 24 - November 22
* Sagittarius - November 23 - December 22
* Capricorn - December 23 - January 20
* Aquarius - January 21 - February 19
* Pisces - February 20- March 20

As you can see from the image at right, they have it wrong. At the beginning of March, for example, the line of sight from the Earth toward the Sun continues on toward the constellation Aquarius. Astrologers say that someone born on March 1^st has the sign of Pisces. For this reason (among others) astronomers regard astrology as an interesting form of entertainment, and not a science.

Questions

1. Use the Seasons and the Zodiac page to answer this question. (http://astro.unl.edu/naap/motion3/zodiac.html) What constellation can the Sun be found in during each month of the year?
   

   January
   February
   March
   April
   May
   June

   July
   August
   September
   October
   November
   December

2. Do the dates of given by astrologers for the signs of the zodiac line up with the dates you found above? What is your opinion about astrology?

3. To fill in the tables in the following problems use the Paths of the Sun Simulator (http://astro.unl.edu/naap/motion3/animations/sunmotions.html) at NAAP. It is found in the Motions of the Sun Module (http://astro.unl.edu/naap/motion3/motion3.html).
   Use the simulator to find the point on the horizon where the Sun rises (Sunrise Azimuth) and the point on the horizon where it sets (Sunset Azimuth). Also, find the altitude of the Sun at transit (when the Sun crosses the local meridian). Finally, figure out how many hours of daylight there are on each of the days given in the table. Hint: The Sun’s altitude at sunrise and sunset is 0°.
   Scarborough, Maine 43.5°N, 70.4°W

   	Sunrise Azimuth	Sunset Azimuth	Sun Transit Altitude	Hours of Daylight
   Vernal Equinox
   Summer Solstice
   Autumnal Equinox
   Winter Solstice

4. Arctic Circle 66.5°N

   	Sunrise Azimuth	Sunset Azimuth	Sun Transit Altitude	Hours of Daylight
   Vernal Equinox
   Summer Solstice
   Autumnal Equinox
   Winter Solstice

5. Tropic of Cancer 23.5°N

   	Sunrise Azimuth	Sunset Azimuth	Sun Transit Altitude	Hours of Daylight
   Vernal Equinox
   Summer Solstice
   Autumnal Equinox
   Winter Solstice

6. Sydney, Australia 34°S, 151°E

   	Sunrise Azimuth	Sunset Azimuth	Sun Transit Altitude	Hours of Daylight
   Vernal Equinox
   Summer Solstice
   Autumnal Equinox
   Winter Solstice

7. How do the Sunrise and Sunset Azimuth values change during the period of time from the Winter Solstice to the Summer Solstice in Scarborough, Maine?
8. How do the Sunrise and Sunset Azimuth values change during the period of time from the Summer Solstice to the Winter Solstice in Scarborough, Maine?
9. How do the Sunrise and Sunset Azimuth values change during the period of time from the Winter Solstice to the Summer Solstice in Sydney, Australia?

10. How do the Sunrise and Sunset Azimuth values change during the period of time from the Summer Solstice to the Winter Solstice in Sydney, Australia?
11. How does the altitude of the Sun in Scarborough, Maine change from the Winter Solstice to the Summer Solstice?
12. How does the altitude of the Sun in Scarborough, Maine change from the Summer Solstice to the Winter Solstice?
13. How does the altitude of the Sun on the Arctic Circle change from the Winter Solstice to the Summer Solstice?
14. How does the altitude of the Sun on the Arctic Circle change from the Summer Solstice to the Winter Solstice?
15. Is the amount of daylight the same year-round or is it seasonal? Explain using Scarborough, Maine as an example.
16. What does the amount of daylight have to do with the seasons?
17. The higher the Sun is in the sky, the more direct are its rays. The more direct rays you get the warmer it is. Why is it warmer at the Tropic of Cancer (23.5°N) than in Maine? (Note: Hawaii is just south of the Tropic of Cancer).
18. Why are the seasons in Australia the opposite of what they are in Maine?
19. What two factors cause the seasons on Earth?

Drawing the Path of the Sun

Use the boxes below to draw the path of the Sun for the Winter Solstice, the Equinoxes (which will be the same) and the Summer Solstice. Label each line carefully since three lines are required in each box. Do this for Scarborough, Maine and for a location on the Arctic Circle. Draw the path of the Sun at the Eastern horizon for sunrise on each of the required dates. Draw the path of the Sun at noon in the Southern horizon box. Finally, draw the path of the Sun for sunset in the Western horizon box. Use the simulator and the data you collected in numbers 3. and 4. The altitude of the Sun for each solstice and equinox should correspond to the data you collected. The sunrise and sunset azimuths should also be approximately correct.

Sunrise	Noon (Sun Transit)	Sunset

Scarborough, Maine, USA

Sunrise	Noon (Sun Transit)	Sunset

Arctic Circle, 66.5°N