THE SOUTHWORTH PLANETARIUM 207-780-4249 www.usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970 "Indonesian authorities advised tourists to observe the total solar eclipse in the Komodo National Park so that when it went dark, they'd be in striking distance when the Dragons panicked." THE DAILY ASTRONOMER Wednesday, March 9, 2016 The Equation of Time -------------------------------------------------------- You might find it helpful to read today's article on the planetarium web-site. http://usm.maine.edu/planet/da-7-december-2015 Each day, we post a new "Daily Astronomer" to this page and we often include illustrations or graphs. Go to this page to see the "Equation of Time" graph. ----------------------------------------------------------- We often say -and, we truly shouldn't- that the Sun rises in the east, attains its highest altitude when crossing the meridian in the due south, and then sets in the west. In fact, Earth is spinning on its axis and the Sun merely appears to rise and set. It is an example of a Ptolemaic illusion, harkening back to the geocentric days when Earth seemed fixed in the cosmic center. Astronomy has humbled us somewhat and so we now know that we revolve around the Sun, not vice versa. We also say -and, here again, we shouldn't- the the Sun crosses the meridian at high noon. Though this is true on occasion, most of the time the Sun does no such thing. Generally, the Sun will either be just east or just west of the meridian at noon time. It will be lagging behind or moving ahead of the "proper" noon position. Therefore, to know precisely when the Sun reaches its highest point due south, we need the "Equation of Time." Here, the term "equation" denotes a corrective factor as opposed to a mathematical formula. Astronomical almanacs often include an "Equation of Time" graph or value table so we can predict precisely when the Sun will reach the meridian, a point called "upper culmination." Inputting this correction is a simple matter of addition or subtraction. For instance, on the 1st day of January, the equation of time is -3.5 minutes, meaning that the Sun will cross the meridian 3.5 minutes after civil noon. Or, one could say that the Sun lags behind "noon" by 3.5 minutes. Around mid February, this equation reaches a local maximum of about 14.5 minutes. Around mid April, the equation of time equals zero, meaning that the Sun crosses the meridian at noon sharp. Then, the Sun starts to move ahead, so that one must introduce a positive factor: Around May 2, the equation of time is 3 minutes, meaning that the Sun reaches upper culmination at 11:57 a.m. The Sun will already be three minutes beyond the meridian by noon time. The equation of time becomes zero again around mid June and again in early September and, once more around Christmas. Between these dates, the equation of time will be alternately positive and negative. This equation of time factor is necessary because Earth is titled on its axis by 23.5 degrees, so the Sun's will be north of the celestial equator during half of the year and south of it during the other half. The angle its apparent path, the ecliptic, makes relative to the celestial equator will also vary. Also, Earth's orbit is slightly elongated, so its distance from the Sun varies from its closest point, perihelion, to its most distant point, aphelion. Consequently, the Sun's "speed" along the ecliptic also changes because Earth moves most quickly around the Sun when it is closest to it and slowest when it is farthest away. If Earth's axis weren't tilted and if its orbit were perfectly circular, an equation of time wouldn't be necessary, as the Sun would cross the meridian at noon every day. But, then, again, what fun would that be?