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Observing the Planets Inferior (inner) planets - Mercury and Venus Superior (outer) planets - Mars, Jupiter, Saturn etc. Rising and Setting Times the time when an object intersects the horizon Transit Time the time when an object crosses the meridian the meridian is the circle on the sky joining north to south Opposition a planet is in opposition when it lies on the opposite side of the sky with respect to the sun (180 degrees away). the full moon is in opposition Conjunction two objects are in conjunction when they have the same value of right ascension planets are in conjunction with the sun, moon and with each other as well as with stars the new moon is in conjunction with the sun Maximum Elongation the inferior (inner) planets are never at opposition they reach a maximum angle from the sun called maximum elongation they undergo superior and inferior conjunctions 1 Conjunction Superior Planet Superior Conjunction Inferior Planet Maximum Elongation Inferior Conjunction Earth Opposition AST101 Lecture 2 AST101 2 Measuring the Distance to the Planets Triangulation trigonometry - triangles of all sizes have the same internal angles you only need to know two angles and the length of a side to solve for a triangle to find the distance to an object observe it from two positions at the ends of a baseline tree C c BC AC a river b A AB B baseline 1. measure length AB 2. measure angles a and b 3. determines AC and BC AST101 Lecture 2 AST101 3 Astronomical distances are large skinny triangles with two angles near 90 degrees and one tiny angle use the Parallax – apparent angular shift of a foreground object with respect to a very distant background object in practice we measure the parallactic angle The Distance to the Planets e.g. two observers on opposite sides of the earth measure the position of Mars at the same time baseline of the triangle is nearly the diameter of the earth Mars’ parallax is about one arcminute at opposition which is easily detectable with modern telescopes The Diameter of the Earth Method of Eratosthenes measure length of shadows at different points on the earth at the same time length of shadow gives altitude angle of the sun gives the angle on the earth’s surface between two locations knowing the distance gives the earth’s circumference and diameter Modern distance measurements come from radar ranging send a radar signal to a planet listen for echo distance = time delay speed of light AST101 Lecture 2 AST101 4 Parallax background stars Mars parallactic angle d s/d s Earth AST101 s - half of the baseline d - distance Lecture 2 AST101 5