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Seasonal Visibility of Stars; and Planet Visibility in 2013-2014 from Positions of Planets in their Orbits -- Robert C. Victor These orbit charts and accompanying data table can be used for plotting the positions of the six inner planets, and determining any planet’s visibility as seen from Earth. In addition to doing the problem set below as a desktop activity, students can “act out” each problem’s situation in the classroom, by having one student represent the Sun, another the Earth, and others the five other planets. Be sure to have all students take a turn at representing the Earth. That student will do more than just stand in place, but will rotate as well, to determine planet visibility at dusk, in middle of night, and at dawn. These two charts of the orbits of the planets, one showing Mercury through Mars, and the other Mercury through Saturn, depict the view as seen from the north side, or “above” the solar system. In these views, the direction of revolution of the planets about the Sun is counterclockwise. The outer circular scale is labeled with values of heliocentric longitude, measured from the Vernal Equinox, or apparent direction of the Sun as seen from Earth at the beginning of northern hemisphere spring. That scale also indicates the directions of the thirteen zodiacal constellations (those in the plane of the Earth’s orbit) from the Sun. The directions of the five first magnitude stars Aldebaran, Pollux, Regulus, Spica, and Antares, as well as the Pleiades star cluster, are also indicated. The outer circular scale should be imagined to be much larger than shown: Earth is one astronomical unit, or 81/3 light minutes from the Sun, compared to stellar distances of many light-years. One light-year is approximately 63,000 astronomical units. On a chart where the Sun-Earth distance (one a.u.) would be represented by one inch, a light year would be represented by one mile. On both orbit charts, the Earth’s orbit is exactly in the plane of the sheet of paper. For each of the other orbits, the portion drawn as a solid curve lies north of or above Earth’s orbit plane. The dotted part of the orbit lies south of or below Earth’s orbit plane. From the north side of the solar system, the Earth’s rotation on its axis also appears counterclockwise. But the axis of Earth does not point at right angles to the plane of the orbit; rather, it tips away from the perpendicular, leaning by about 23.4° toward the top of the chart or the 90° mark on the circular scale. Using both orbit charts and the data table, try working out the answers to these questions: (1) Why is the Pleiades star cluster visible all night around November 20? Where (in what direction in the sky?) would you expect to see it at nightfall? In the middle of the night? At dawn’s first light? Why can’t you see the cluster for several weeks around May 20? (2) On what approximate date each year is Aldebaran visible all night? Give approximate date of all-night visibility for Pollux; Regulus; Spica, Antares. (3) On what approximate date each year is Earth heading toward Antares and away from Aldebaran? On that date, Antares is visible at (dusk or dawn?) about 90 degrees from the Sun, while Aldebaran is visible at (dusk or dawn?), about 90 degrees from the Sun. (4) In which month would a Last Quarter Moon appear near the star Spica? Hint: The Last Quarter Moon occurs when the Moon appears 90 degrees or a quarter-circle west (clockwise in this top view) of the Sun. (5) Which constellation is hidden beyond the Sun for about the first month of Northern Hemisphere summer? (6) At the end of which month in 2013 will Venus reach its greatest angular separation from the Sun in the afternoon and evening sky? In what phase will Venus appear then? Try to observe it through a telescope in the daytime around then, and with telescopes and binoculars in following weeks. (7) What alignment involving Sun, Venus, and Earth will occur on January 11, 2014? Notice Venus will be located in the portion of its orbit plotted as a solid curve, rather than dotted. At the alignment taking place on Jan. 11, will Venus pass north, or south, of the Sun’s disk? Just over 19 months earlier, on June 5, 2012, Venus passed directly in front of the Sun’s disk, causing a transit of Venus, which won’t happen again until December 10, 2117. During the weeks leading up to and following the event on January 11, 2014, what will be the phase of Venus? (8) Which planet will be visible all night on January 5, 2014? In which constellation does it appear? (9) Which planet will be visible all night on April 8, 2014? In which constellation does it appear? Which bright star will appear very close to the Full Moon on the night of April 14, 2014 with the same planet nearby? Later that night, there will be a total eclipse of the Moon, as the Moon passes through the Earth’s shadow. (10) Saturn will be visible all night just before the middle of what month in 2014? (11) Which four planets will be visible at dusk (soon after sunset) for most of May 2014? List the four planets in order of their positions in the sky, from western horizon to eastern horizon. As the Earth rotates, which planet will set first? Second? Third? Last? (12) Which two planets will appear close together in our sky on August 18, 2014? What time of day will they be seen, at dusk or at dawn? (13) Which other planets will form a fairly close pair later that month? Will the event be seen at dusk or at dawn? Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing sky watching opportunities for school children in and around Palm Springs.