StarFinder User's Guide: Whole Sky Maps
StarFinder User's Guide: Whole Sky MapsA whole sky map, like the example below, is typically drawn as a circle with directions (North, South, etc.) indicated around its perimeter. The edge of the circle corresponds to the horizon, the center of the circle corresponds to directly overhead. Stars drawn near the edge of the map correspond to stars seen just above the horizon, in the direction indicated (N for North; S for South, etc.). Conversely, stars drawn near the center of the circular map correspond to stars seen high overhead.
Imagine looking up at the star map as you hold it at arm's length straight above your head, with the N at that top of the map pointing in the direction of north. The stars drawn near the center of the map correspond to the stars you would see looking past (or through) the map to the sky directly overhead. Notice that in this orientation, holding the map high and looking up at it, east (E) and west (W) are oriented properly. This is opposite of a typical map, since typical maps are of the Earth's surface and one typically looks down on those (just as one typically looks down on the Earth). With a star map you typically hold it at arm's length next to the stars you are trying to identify, so typically you are looking up (more-or-less) at the map (since you are typically looking up, more-or-less, at the stars).
Now imagine holding the star map at arm's length with the S at the bottom as you look just above the southern horizon. The stars drawn near the bottom edge of the map correspond to the stars you would see looking past (or through) the map to the sky just above the southern horizon. Alternatively, if you want to look towards the horizon in a different direction, say, for example, the west, simply turn the map so that the west (W) is at the bottom.
Star maps are great to take outside when you are looking at the stars, but they have some drawbacks.
First, your latitude, i.e., how far north or south of the equator, affects where the stars appear in your sky, if at all. If, for example, you stood at the North Pole, the north star, Polaris, would be directly overhead. If, on the other hand, you were only a bit north of the equator, Polaris would be just above the horizon to the north. If you were south of the equator, you would not ever see Polaris. Latitude is measured in degrees, from zero degrees at the equator to 90 degrees north (the North Pole) and 90 degrees south (the South Pole). If the map was drawn for a latitude within about 10 degrees or so of your latitude it should work just fine.
Second, because the Earth rotates once per day, the stars move across the sky as time passes just as the Sun does. If the map is drawn correctly for the early evening, it will become increasingly inaccurate as the night gets later, and be quite wrong in the few hours before sunrise.
Third, because the Earth orbits the Sun, the stars that we see, say directly overhead at midnight, are not the same in January as they are in April, June, or September. In fact, the stars that you see directly overhead in January at midnight are the stars that you see just above the western horizon at midnight in April. Between the Earths rotating and orbiting you really need about four different star maps, depending upon the month of the year and the time of night. Alternatively, someone invented a planisphere or star finder which is a star map on a rotating wheel that you can adjust for the date and time of your observing.
Unfortunately, even these adjustable star maps share some drawbacks with the non-adjustable star maps. The Moon, and the other planets of our solar system, move around in the sky from night to night (the Moon), or from month to month and/or year to year (Venus, Mars, Jupiter and Saturn). It is impractical to make a perminent map with these moving objects on it.
Copyright (c) 2001 by G. L. Deitrick.
Permission to use is granted provided appropriate acknowledgement is given.