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As an angle of 1° corresponds to 4 minutes in time (360° = 24 hours), we need therefore four extra minutes of diurnal motion to see the Sun back on the same meridian again, making the duration of one rotation just 24 hours exactly (on the average, ignoring small seasonal variations, see equation of time)

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The sun moves about 1° per day eastwards over a great circle known as the ecliptic (which is 360° or a full circle in one year, the annual motion of the Sun). The reason for this is that the Sun does not stand still on the celestial sphere, and the stars do (except for a tiny proper motion).

By the following night the difference will be 8 minutes, and so forth with every following night (or day). On the next night a particular star will rise again, but with our normal clocks running a 24-hour 0-minute cycle, it will do so slightly less than 4 minutes earlier. This is why stars will rise in the east and set in the west, (unless a star is circumpolar). This is the diurnal motion ( daily motion). The celestial sphere and all objects on it appear to rotate from east to west around the celestial poles in the same time. The Earth rotates from west to east around its axis in slightly less than 24 hours. One can also locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere.

Similarly, the celestial sphere is divided by projecting the equator into space. The equator divides the earth into the Northern Hemisphere and the Southern Hemisphere.

Please note that we do not offer subscriptions for more than one year at a time.Celestial hemispheres Gift subscriptions are welcome and recipients are sent a notification card.Ī Sky Calendar subscription is $12.00 for a year. A subscription may start anytime and consists of twelve issues. The Sky Calendar is published in loose-leaf form and mailed quarterly (Feb-Mar-Apr May-Jun-Jul Aug-Sep-Oct Nov-Dec-Jan). Astronomer George Lovi, in his Ramblings column for Sky & Telescope, called it 'one of the most approachable, understandable, and friendly observing guides ever offered.' In his review column in the latter, Phillip Morrison determined it to be 'a bargain entry into direct observation of the the changing order visible in the heavens.' In an annual summary of astronomy publishing (in Mercury magazine), the Astronomical Society of the Pacific gave its Special Commendation to the Sky Calendar. It has since had favorable reviews not only from astronomical publications, such as Sky & Telescope and Mercury, but also from School Library Journal and Scientific American. The Sky Calendar has received awards for excellence in educational journalism from the Educational Press Association of America. Both the sky map (on the reverse side) and the calendar have appeared in issues of Science and Children, a journal of the National Science Teachers Association. Not only is it enjoyed by thousands of paid subscribers, but it is used (with permission) by classroom teachers with their students, by planetariums and astronomy clubs, and by park interpreters for audiences at sky talks. The Sky Calendar has evolved into one of the nation's most highly illustrated, easy-to-follow guide to sky events. The sky maps are designed for use at a convenient time in mid-evening, for a latitude useful for the entire continental U.S. The reverse side consists of a simplified star map of the month's evening sky. Diagrams in the boxes invite the reader to track the moon's rapid motion past the planets and bright stars of the zodiac, as well as to follow the more leisurely pace of the planets in their gatherings with bright stars and other planets. As its name implies, the sheet for each month takes the form of a calendar. The Abrams Planetarium Sky Calendar promotes skywatching for people of all ages.