lab1 - AST 3722C Spring 2008 Lab#1 Constellations...

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AST 3722C - Spring 2008 Lab #1: Constellations, Planisphere, Motion of the Sky. Due before class on February 5. In this lab you will get a more concrete picture of how the sky works. It will help you better understand the math we have been using in class to describe such things as equatorial coordinates, horizon coordinates, hour angles, and availability of objects. Your goals for this lab are as follows: Get a sense for approximate angular distances. Watch the sky over the course of 1 to 2 hours to visualize the motion of the sky over the course of a night. Learn some basic constellations that will orient you to the cardinal directions, the celestial equator, and the ecliptic. Understand how to use a planisphere, and how the hemispherical sky is mapped to a flat plane. The only equipment you’ll need for this lab is: your eyes, a clock, a planisphere, a calculator, and maybe a flashlight. Some of this assignment has to be done while at lab, but some of it can be at home later. Planisphere basics: The planisphere consists of two parts, the inner, rotatable wheel and the outer “frame”. The wheel has the brightest stars marked on it and uses the equatorial coordinate system. The frame has been pre-cut to represent the horizon for your latitude region on Earth. (A planisphere that is cut for a different latitude region will still work for you down here in Florida, just not nearly as well.) By lining up the month and day on the wheel with the time printed on the frame, we are effectively converting between the equatorial and horizon coordinate systems. To make such a conversion, we need the time and place (or really, the latitude) of the observations; the frame itself takes care of the place and we just dial in the time. The horizon is marked with north, east, and west on the front side and with south, east, and west on the back side. Where is the zenith? It’s not at the metal pivot – that’s the Celestial Pole. In the front side cutout area, the zenith is about one-third of the way down from the top. In the back side cutout area, it’s just about off the top edge. Keep in mind the orientation of the your sky as it is flattened onto the planisphere. For example, when you’re facing North, and looking at the front side of the planisphere, an imaginary line heading up from the pivot corresponds to your meridian heading up from Polaris, toward your zenith, and then falling back down in altitude toward your southern horizon. Don’t forget that your zenith corresponds to a Declination equal to your latitude, about 28 . 6 . Why does the cutout area have a funny shape? Why aren’t the distances to the horizons the same in each cardinal direction? Well that’s because we have to put a hemispherical sky onto a flat plane – it’s similar to the problem of putting a map of the whole world on a piece of paper. Things get distorted. You might notice that on the front side of the planisphere, the southern constellations are getting stretched out more. This particular kind
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