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Motions Astronomy 1-1 Lecture 02-1 Motions in the Sky When viewed from the north pole of the solar system Earth moves counterclockwise about the Sun and rotates counterclockwise about its own axis Moon moves counterclockwise about the Earth and rotates counterclockwise about its own axis Astronomy 1-1 Lecture 02-2 Diurnal Motion The Daily Motion of the Sun and the rotation of the stars http://lasp.colorado.edu/~bagenal/1010/graphics/apparent_motion.html Astronomy 1-1 Lecture 02-3 Night View At the same time every night, stars appear in different positions due to the motion of the Earth about the Sun. Astronomy 1-1 Lecture 02-4 Definitions Zenith Point directly overhead Meridian A line going from North Pole through Zenith to the South Pole Astronomy 1-1 Lecture 02-5 Daily Time Keeping Our day is based on the Solar Day Apparent Solar Day Time for Sun to go from overhead to overhead Depends upon the season Mean Solar Day Defined to be 24 hours Our clocks are based on this Sidereal Day Time for Earth to make one full rotation about its own axis It has a value of 23h 56m 4.1s Astronomy 1-1 Lecture 02-6 Daily Time Keeping Both the Apparent and Mean Solar day often do not correspond to an Earth rotation, about its own axis, of 360 degrees. The Sidereal day corresponds to an Earth rotation of just 360 degrees. This is the correct rotation period of the Earth. Astronomy 1-1 Lecture 02-7 Daily Time Keeping Apparent Solar Day Sidereal Day Astronomy 1-1 Lecture 02-8 North Celestial Pole A point in the northern hemisphere that stars apparently circle Currently the north celestial pole points towards Polaris This was not always so, and will not be so in the future The Earth's rotation axis is affected by external forces The axis orientation changes with respect to the stars, it precesses A complete precession takes ~26,000 yrs Noticed by ancient astronomers Similar to rotating gyroscope Astronomy 1-1 Lecture 02-9 Precession of Earth’s Rotation Axis Also known as the precession of the equinoxes One such complete precessional cycle in a period of approximately 26,000 years Being a nonspherical shape, the gravitational tidal forces of the Moon and Sun apply torques as they attempt to pull the equatorial bulge into the plane of the ecliptic Astronomy 1-1 Lecture 02-10 Seasons Earth's axis of rotation is tilted at 23.5 degree with respect to orbital plane of Earth around Sun This tilt causes the hemispheres to alternate in the amount of our Sun’s light and heat they receive through the year This is responsible for the seasons that are experienced Holds for all planets Astronomy 1-1 Lecture 02-11 Northern Hemisphere Summer More daylight hours, more direct sunlight In Northern Hemisphere summer, Earth is actually slighter further from the Sun than in the winter Earth is CLOSEST to the Sun (91 million miles) in Winter ~January 3 Earth is FARTHEST from the Sun (94 million miles) in Summer ~July 4 Astronomy 1-1 Lecture 02-12 Celestial Sphere Coordinate system based on the Earth’s perspective A sphere whose equatorial plane coincides with the equatorial plane of the Earth The north celestial pole is coincident with the north pole of the Earth Plane of sun's movement not the same as equatorial plane The two planes are tilted with respect to each other at an angle of 23.5 degrees The path of the sun is called the ecliptic Astronomy 1-1 Lecture 02-13 Celestial Sphere Astronomy 1-1 Lecture 02-14 Coordinates on the Celestial Sphere They are: Declination (latitude) It is measured in units of degrees, minutes, and seconds. This is the angular distance above or below the celestial equator. Right Ascension (longitude) It is measured in units of hours, minutes, and seconds. Its origin is the vernal equinox point and is measured to the east. Astronomy 1-1 Lecture 02-15 Major Points on the Ecliptic Four major points along the ecliptic: VERNAL EQUINOX Point where Sun crosses the equatorial plane from below to above. Occurs around March 21. SUMMER SOLSTICE Highest point above equatorial plane. Occurs around June 21. AUTUMNAL EQUINOX Point where Sun crosses the equatorial plane from above to below. Occurs around Sept 23. WINTER SOLSTICE Lowest point below equatorial plane. Occurs around Dec 21. Astronomy 1-1 Lecture 02-16 Time Keeping II As with the day, there are several types of years. SIDEREAL YEAR True orbital period of the Earth around the Sun. 365.2564 Mean Solar Days TROPICAL YEAR Time for passage from one vernal equinox to the next. 365.2422 Mean Solar Days GREGORIAN YEAR Calendar based on tropical year. This is the one that is used. Error of only 1 day in 3300 years. 365.2425 Mean Solar Days Astronomy 1-1 Lecture 02-17 Calendar If the calendar that is used were based on the sidereal year, the seasons would slowly shift with respect to the calendar because the Earth's rotation axis precesses. This precession causes the relative facing of the Earth towards the Sun at a given point on the Earth's orbit to change from year to year. Astronomy 1-1 Lecture 02-18 Planetary Motions Inferior Planet Planet closer to the Sun than the Earth. Mercury and Venus Superior Planet Planet further from the Sun than the Earth. Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto Astronomy 1-1 Lecture 02-19 Important Points Along a Planet's Orbit Inferior Planets Inferior Conjunction Planet is directly between Earth and Sun. Superior Conjunction Sun is directly between Earth and planet. Maximum Elongation Point on the planet's orbit where it has the largest angular distance between it and the Sun as viewed from Earth. Astronomy 1-1 Lecture 02-20 Important Points Along a Planet's Orbit Superior Planets Opposition Earth is directly between the planet and the Sun. Conjunction Sun is directly between the planet and Earth. Quadrature Planet is see at 90 degrees with respect to the Sun as viewed from Earth. Astronomy 1-1 Lecture 02-21 Mars’ Interesting Motion Mars in its motion seems to go backwards What is the cause of this apparent motion? Astronomy 1-1 Lecture 02-22 Retrograde Motion This apparent temporary backward motion of a superior planet along its orbit is caused by the Earth's faster motion along its own orbit. The Earth overtakes the planet. Astronomy 1-1 Lecture 02-23