1. Astronomical Coordinates Summary
Relative to Observer
Local Horizon
Local Meridian
Local Zenith
Altitude-Azimuth Coordinates
Pro: Easy to record for instantaneous position
Con: Objects change angles (coordinates) throughout the night
Con: not easy to compare measurements made by different observers

2. Ancient Alt.-Az. Observatory
Chankillo, Peru
2300 years old
Marks positions of sunrise throughout the year

3. Last Class Summary: Astronomical Coordinates
Fixed to rotating Earth
Celestial Equator
Right Ascension: Measure from Vernal (March) Equinox
Declination: measure from Celestial Equator
Celestial Coordinates
Pro: stars have fixed coordinates
Pro: easy to compare measurements made by different observers
Note: Sun changes celestial coordinates throught the year as it follows the ecliptic through the Zodiac
Note: Require a clock at each observatory for coordinate system reference.

4. Time of the Seasons
We have seasons because of the tilt of the Earth’s axis.
The Ecliptic appears tilted 23.5° from the Celestial Equator because Earth’s axis is tilted 23.5° from the vertical to the plane of its orbit
Because of this tilt the Sun appears half the year North of the Equator and half the year South of the Equator
The seasons are defined based on the location of the Sun compared to the Celestial Equator
This figure represents the earth orbiting the sun over one year. It illustrates the fact that the earth’s axis is tilted 23.5 degrees.

5. Summer On the first day of Summer:
The Sun is at its farthest distance North of the Celestial Equator (23.5° N declination). The is known as the Summer Solstice.
The Northern Hemisphere is tilted towards the Sun & we experience the longest day of the year (about June 21).
The Southern Hemisphere is tilted away from the Sun. This is the beginning of Winter and the shortest day of the year there.
Sun rise and sets at its most northerly
The word solstice comes from the Latin meaning “Sun standing” because it is at the solstices that the Sun stops moving either further North or further South and begins to turn around.
What do we experience during a typical summer day that is different from a typical winter day?
- temperatures are higher in the summer. Why?
- there are more hours of daylight in the summer. Why?
During our summer months what season is it in the Southern Hemisphere (south of the equator)?
- it would be winter during our summer. Why?

7. Why it’s hotter in the Summer and colder in the Winter
Direct versus Indirect Sunlight.
Imagine standing in front of a fire on a cold night. Your hands are cold so you extend them towards the fire palms outward. Why? You want to receive as much heat as you can from the fire. You know that you intercept more heat energy by holding your hands this way rather than holding them on edge.
The Season Lag
Usually the hottest days of the year are in July and August not in June when the Sun is at its most northerly.
It takes time for the oceans and land to heat up.
The hottest time of day isn’t at noon either but an hour or two later.
At the summer solstice, the North gets more direct sunlight
and more hours of sunlight. At the same time it is winter in the South
(less direct sunlight, fewer hours of daylight).

8. In the winter months the Sun is lower in the sky for fewer hours and the sunlight is spread over a larger area.

9. “But I thought the reason it was hotter in the summer was because the Earth was closer to the Sun in the Summer.”
If this were true then why is the Earth closest to the Sun in January and farthest from the Sun in July? and
Why does the Southern Hemisphere have seasons that are opposite that in the Northern Hemisphere?

10. Winter On the first day of Winter:
The Sun is at its farthest South of the Celestial Equator (23.5° S declination). This is known as the Winter Solstice.
The Northern Hemisphere is tilted away from the Sun & we experience the shortest day of the year (about December 21).
The Southern Hemisphere is tilted towards the Sun. This is the beginning of Summer and is the longest day of the year there.
Sun rise and set is at its most southerly.

11. Spring and Autumn
The Sun crosses the Celestial Equator heading North on the first day of Spring (the Vernal Equinox, about March 21).
It crosses the Celestial Equator again heading South on the first day of Autumn (the Autumnal Equinox, about September 21).
In both the Northern and Southern Hemisphere days and nights are of equal length

12. Seasons Interactives Seasons Interactive I - McGraw Hill
Seasons Interactive II - University of Nebraska Lincoln (UNL)
Sun Motion – UNL
Seasons Interactive III - Iowa State

13. Summary – The Seasons
The rotation of the Earth on its axis determines the length of the day
The orbit of the Earth around the Sun (revolution) determines the length of the year
The tilt of the Earth with respect to the ecliptic causes the seasons.

14. Summary - Equinoxes
The two equinoxes occur when the Sun crosses the Celestial Equator
In March, as the Sun is heading North, and
In September, as the Sun is heading South
Equinox means “equal night”
Length of night is the same as the length of daylight for the equinoxes
The Sun will rise due East and set due West.
For an observer on the Earth’s equator
The sun will be at the observer’s zenith at local noon.
For an observer on a pole: the Sun circles the observer on the horizon…16 hours from half-to-all or half-to-gone.

15. Equinoxes At lat. = 20 On equator, lat. = 0 At lat. = 50
At pole lat. = +90

16. Summary - Solstices
The two solstices occur when the Sun reaches its extreme North or South positions in the sky.
December Solstice: farthest South
June Solstice: farthest North

17. Solar Altitude Variation Over Six Months

18. Analemma Earth's axis is tilted 23.5 degrees
Earth's orbit is elliptical, not circular

19. Analemma Interactive http://www.analemma.com/SunGraph/index.html
APOD - Analemma Movie over New Jersey

20. Analemma for Greenwich, England Latitude +51