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

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

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.

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.

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?

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).

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

“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?

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.

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

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

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.

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.

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

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

Solar Altitude Variation Over Six Months

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

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

Analemma for Greenwich, England Latitude +51