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Explore the Celestial Sphere, star paths, constellations, planetary motions, and historical celestial theories in astronomy. Learn about Earth's rotation, the Solar System, and the moon's phases and motions.
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Astronomy Celestial Observations
The Celestial Sphere All celestial objects, (sun, stars, moon, planets), from Earth appear to be on the surface of an imaginary sphere surrounding Earth. This imaginary sphere is called the Celestial Sphere.
Celestial Observation Terms Horizon = where celestial sphere meets Earth. Zenith = point on celestial sphere directly above observers position. Altitude = objects distance, in degrees, above horizon. Azimuth = objects distance, in degrees, measured clockwise from due north position (due north is 00 azimuth).
Star Paths All celestial objects appear to move from east to west across the sky in an arc. Why? Circumpolar stars move in counterclockwise circles between the northern horizon and Polaris and never set.
Rotation The apparent daily motion of celestial objects is due to Earth’s rotation. Rotation is the turning of an object on its axis. Celestial objects are not actually moving from east to west across the sky, rather the earth is rotating under them.
Revolution Revolution is the movement of one celestial object around another. The path along which an object travels during a revolution is called an orbit.
What are Constellations? Constellations are stars that appear to be grouped in patterns that form the outlines of “things” (people, animals, objects). The night sky is divided into 88 constellations. Circumpolar constellations are those constellations that are always visible as they never set below the horizon.
Star Shifts Constellations shift, rise and set, (similar to the rise and set of the Sun) during a nightdue to the rotation of the Earth. Constellations in the sky slowly change from one night to the next as a result of the revolution of Earth around the sun. Constellations rise approximately 4 minutes later each night.
Planetary Motions • On a daily basis planets and stars appear to move from east to west. • Over long periods of time the planets appear to move eastward relative to the stars behind them.
Geocentric Theory: this theory, put forward by the ancient Egyptian astronomer Ptolemy, states that the Earth is the center of the universe. Heliocentric Theory: this theory, put forward by the Polish scientist Copernicus, states that the Sun is the center of the universe. Celestial Models
Orbital Forces Johann Kepler developed the “Laws of Planetary Motion” which states that planets orbit in ellipses. Unlike a circle the center of an ellipse consists of two fixed points called foci. Kepler’s First Law: The orbits of the planets around the Sun are ellipses, with the Sun at one of the foci.
Kepler’s Laws Kepler’s second law states that: An imaginary line joining a planet to the Sun will sweep over equal areas in equal periods of time. Perihelion = when the planet is closest to the Sun (moving the fastest). Aphelion = when the planet is farthest from the Sun (moving the slowest).
Evidence of Earth’s Rotation • Foucault Pendulum:the greater the latitude the greater the hourly change in direction. • Coriolis Effect:is the tendency of matter moving across the Earth’s surface to be deflected from a straight-line path. • Other evidence includes the day night cycle, star paths and satellite photos.
Apparent Motions of the Sun • Due to the rotation of the Earth, the Sun appears to move in an arc across the sky from east to west. • The tilt of the Earth’s axis, 23.50, causes the location of the direct rays of the Sun to change in a cyclic pattern.
Change of Seasons • Three factors cause the seasonal changes: • Earth’s revolution around the sun • Tilt of the Earth’s axis • Parallelism of the axis
Time and Earth Motions • Sidereal day is the time period for Earth to make one complete rotation on it’s axis (23 hrs, 56 min, 4 sec.). • Apparent solar day is about 4 min longer (based on solar noon to solar noon) to account for the additional rotation needed as a result of Earth’s revolution around the sun. • Since our orbital speed varies, therefore the apparent solar day varies, the mean solar day (24 hrs) is used for daily timekeeping.
Moon Motions • Moon orbits earth once every 27 1/3 days. • The barycenter is the gravitational center the Earth and Moon revolve around (1700km beneath surface of Earth). • Sidereal month (27 1/3 days) is time for the Moon to revolve around the Earth. • Synodic month (29 ½ days) is the time for the Moon to complete a cycle of phases.
Phases of the Moon • The illuminated portion of the moon that faces Earth varies in a cyclic pattern called phases. • New moon = when the entire illuminated portion is facing away from Earth. • Waxing period = when right hand portion seems to grow more lighted. • Full moon = when the entire illuminated portion is facing toward Earth. • Waning period = when the left hand portion shrinks in illumination.
Eclipses • Solar eclipse occurs when the Moon passes between the Earth and the Sun. • Umbra is the part of the Moon’s shadow in total darkness • Penumbra is the part of the Moon’s shadow in partial darkness. • Annular eclipse occurs when the Moon is at apogee and the outer edge of the Sun is visible. • Lunar eclipse occurs when the moon passes through the Earth’s shadow.
