NATS From the Cosmos to Earth A model of the celestial sphere shows the patterns of the stars, the borders of the 88 official constellations, the ecliptic, and the celestial equator and poles.

NATS From the Cosmos to Earth Latitude - lines of latitude parallel to Earth’s equator - labeled north or south relative to equator - from 90º N to 90º S Longitude - lines of longitude extend from North Pole to South Pole - by international treaty, longitude 0 (the prime meridian) runs through Greenwich, England Latitude and Longitude Dallas: latitude = º N longitude = 96.78º W

NATS From the Cosmos to Earth A circumpolar constellation never rises or sets - they are always visible. Your latitude determines the portion of the celestial sphere visible in your sky. (a) A Northern Hemisphere sky. (b) A Southern Hemisphere sky. At what latitude would you see the entire sky?

NATS From the Cosmos to Earth The Earth's rotation causes stars to trace daily circles around the sky. The north celestial pole lies at the center of the circles. Over the course of a full day, circumpolar stars trace complete circles, and stars that rise in the east and set in the west trace partial circles. Here, the time exposure lasted about 6 hours - we see only about one-quarter of each portion of the full daily path. Star Trails The Northern Hemisphere The Southern Hemisphere

NATS From the Cosmos to Earth Finding the Celestial Poles You can always find north using the North Star. Polaris can be found using the big dipper. Draw a line through the two “pointer” stars at the end of the big dipper and follow it upwards from the dipper about four outstretched hand’s width. The big dipper is circumpolar in the US so is always above the horizon. The south celestial pole can be found using the Southern Cross. There is no “South Star”

NATS From the Cosmos to Earth The Big and Little Dippers

NATS From the Cosmos to Earth Motion of the Night Sky Animation

NATS From the Cosmos to Earth The height in degrees of the north star above the horizon is the same as your latitude.

NATS From the Cosmos to Earth  The angle  between the horizon and Polaris is the latitude of the observer. If Dallas is at 33º latitude, where is Polaris in the sky? Where is it at the Equator?

NATS From the Cosmos to Earth Angular Size Distances in the sky measured by angular distance: Minute of arc = 1/60th of a degree Second of arc = 1/3600th of a degree Angular diameter - angular distance from one side of an object to the other

NATS From the Cosmos to Earth Earth travels around the sun (orbits) once per year in the same direction it rotates. It’s orbit is not quite a perfect circle - it is elliptical. The location in the orbit of the minimum and maximum distances from the Sun are called perihelion and aphelion. The plane of the orbit is called the ecliptic. Revolution

NATS From the Cosmos to Earth Ecliptic Plane The Earth’s axis is currently tilted 23.5º to the ecliptic. It varies over time between 22º and 25º due the the gravitational forces from Jupiter and the other planets. Earth’s Axial Tilt

NATS From the Cosmos to Earth The axis remains at the same tilt angle - pointed at Polaris - throughout the orbit because of conservation of angular momentum. The ecliptic plane is the plane of the Earth’s orbit. Looking from the Earth, it is the apparent path of the Sun (and planets) in the sky.

NATS From the Cosmos to Earth The Relationship of the Celestial Equator and the Ecliptic Plane

NATS From the Cosmos to Earth The Zodiac The Sun appears to move steadily eastward along the ecliptic, through the constellations of the zodiac. As Earth orbits the Sun, we see the Sun against the background of different zodiac constellations at different times of year. For example, on August 21 the Sun appears to be in the constellation Leo. Defines astral calendar.

NATS From the Cosmos to Earth Sun’s Path Through the Zodiac

NATS From the Cosmos to Earth Celestial Sphere The apparent Sphere of the sky Celestial Poles The points about which the celestial sphere appears to rotate Celestial Equator Projection of the Earth’s equator on the celestial sphere EclipticApparent annual path of the sun on the celestial sphere

NATS From the Cosmos to Earth Seasons occur because even though the Earth's axis remains pointed toward Polaris throughout the year, the orientation of the axis relative to the Sun changes as the Earth orbits the Sun. Around the time of the summer solstice, the Northern Hemisphere has summer because it is tipped toward the Sun, and the Southern Hemisphere has winter because it is tipped away from the Sun. The situation is reversed around the time of the winter solstice when the Northern Hemisphere has winter and the Southern Hemisphere has summer. At the equinoxes, both hemispheres receive equal amounts of light.

NATS From the Cosmos to Earth Why Does Flux Sunlight Vary Animation

NATS From the Cosmos to Earth Antarctica June 21 December 21

NATS From the Cosmos to Earth In the summer hemisphere, the sun follows a longer and higher path. The sunlight is more intense - more direct and more concentrated. In the winter hemisphere, the sun follows a shorter and lower path. The sunlight is less direct and less intense. Why are the warmest days one to two months after summer solstice?

NATS From the Cosmos to Earth The Relationship of the Celestial Equator and the Ecliptic Plane The solstices are when the celestial equator and the ecliptic have the greatest separation - when the Sun is highest in the sky. The equinoxes are when ecliptic and the celestial equator cross - when the Sun crosses from north/south of the celestial equator to south/north of the equator Northern spring equinox is when the Sun passes from south of the celestial equator to north.

NATS From the Cosmos to Earth Why are the seasons more extreme in the Northern hemisphere?

NATS From the Cosmos to Earth 1. Most of Earth’s land mass in in the Northern Hemisphere. Water takes longer to heat or cool than soil or rock. The water temperature remains relatively constant, thereby moderating the climate. 2. Earth is slightly farther from the sun during northern summer solsitce - moves slower in its orbit so summer/winter is days longer/shorter. This effect is more important then the slightly more intense sunlight due to Earth being closer/farther away.

NATS From the Cosmos to Earth 1. The Arctic Circle (66.5 degrees N) 2. Tropic of Cancer (23.5 degrees N) 3. The Equator 4. The Tropic Capricorn (23.5 degrees S) 5. The Antarctic Circle (66.5 degrees S) Five Major Circles of Latitude What is special about these latitude circles?

NATS From the Cosmos to Earth The Arctic and Antarctic Circles - One day a year the sun shines all day and one day a year it doesn’t shine at all. Tropic of Cancer (Capricorn) - The sun is never directly overhead at higher latitudes. Five Major Circles of Latitude

NATS From the Cosmos to Earth Daily Paths of the Sun at Equinoxes and Solstices At the equator At the north pole At the tropic of Cancer At 40º latitude

NATS From the Cosmos to Earth Celestial Coordinate System In this coordinate system the position of a star is specified by the right ascension and the declination. This system is used more than any other by astronomers. Equatorial System –Declination -  - Angle of a star North or South of the Celestial Equator - Measured in Degrees –Right Ascension -  - Measure of Angular Distance of a Star East of the Spring Equinox - Measured in Hours, Minutes, Seconds

NATS From the Cosmos to Earth Coordinate Systems Geographic Celestial Latitude - lines of latitude parallel to Earth’s equator - labeled north or south relative to equator - from 90º N to 90º S Longitude - lines of longitude extend from North Pole to South Pole - by international treaty, longitude 0 (the prime meridian) runs through Greenwich, England Declination - lines of declination parallel to celestial equator - labeled positive or negative relative to celestial equator - from - 90º to +90º Right ascension - lines of right ascension run from north celestial pole to south celestial pole - by convention 0 runs through spring equinox - measured in hours, minutes and seconds east of spring equinox - one hour is 15º

NATS From the Cosmos to Earth