USING SETTING CIRCLES Last Updated: 17 February 2010
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USING SETTING CIRCLES Last updated: 17 February 2010 Setting Circles on the ETX / LX Telescopes From: Clay Sherrod (Clay Sherrod) Introduction - To newcomers to astronomy there is a certain "mystique" surrounding those numbered "dials" which come as standard equipment on a quality equatorial telescope. A telescope equipped with an equatorial mount is one which can be correctly "aimed" at celestial north (Figure 1) thereby allowing the easy tracking of celestial objects as the earth turns on its axis. Once so aligned, the telescope - through the motions of the equatorial mounting - is able to "keep up" with the object through just one slight motion compensating for the earth's rotation. Believe it or not....faint objects in the sky CAN BE located without the use of a microprocessor and a hand paddle! To allow an observer from anywhere in the world to access (without an AutoStar) an object that might be too faint to see with the naked eye, the sky is mapped much like the Earth's globe. On Earth, the sphere of our planet is marked through latitude and longitude and all permanent objects on its surface are mapped accoringly. To understand the coordinates of the sky - declination and right ascension - you must first understand how they correlate with these similar positional measurements on Earth. Because they are so distant and do not appear to change, it has allowed astronomers over the centuries to catalog and accurately map the stars, the many clusters, galaxies and nebulae within them into catalogs and star atlas. So, this method of charting the sky into right ascension and declination allows us to map the celestial objects on the celestial sphere just like latitude and longitude on the sphere of our Earth have allowed us to geographically specify the location of each and every city, mountain, lake, and even HOUSE on the surface of our planet! ---------------------------- Latitude and "Declination" - Latitude is the NORTH/SOUTH measurement of the earth, in degrees from the equator (0°) to the north pole (+90°) or to the south pole (-90°). As an example, the city of Arlington, Virginia is located from the equator toward the north pole at about Latitude +37°. By contrast, the southern hemisphere city of Melbourne, Australia is positioned from the equator to the SOUTH pole at about Latitude -37°. No matter east-to-west on earth, the measurement of latitude remains the same. In the mapping of the sky, the equivalent (north-to-south) measurement of latitude is called "declination" and is measured - like on the earth's sphere - from the celestial equator (0°) north (positive degrees) and south (negative degrees). Thus, "Celestial North" is +90° while "Celestial South" measures -90°. If an imaginary line is drawn through the earth, extending through both the south pole and the north pole, this line would point in either direction to the "Celestial Poles," south and north, respectively. In the northern hemisphere, this imaginary line extends spaceward toward the relatively bright star POLARIS in the constellation of Ursa Major, and is used as a guide for aligning telescope equatorial mountings as described in "Kochab's Clock," Part 5 of the "ETX Enhancement Guide..." series on this web site. Unfortunately, no such bright star exists at or near the south celestial pole. No matter where you are on earth, from an east-west standpoint, the measurement of Latitude remains the same. "Figure 2" shows lines of declination - similar to "latitude" - in the sky in relation to the position and angle of the Earth. Longtitude and "Right Ascension" - On Earth the mapping of EAST/WEST direction is in degrees of Longitude. Arbitrarily beginning at the Royal Observatory in Greenwich, England (0° Longitude), longitude is measured eastward (positive degrees) and westward (negative degrees). At the International Date Line, both "meet" and equal a common 180°. For example Memphis, the southern city in the United States, is nearly one- quarter the way WESTWARD around the globe from Greenwich, at a latitude of -91°; going the other direction from Greenwich, the Russian city of Yartsavo is almost one-quarter EASTWARD, at +90°. As with east-west direction to measure latitude, it does not matter if you are north or south of the equator to measure longitude. The measurement of east-west direction of the sky is markedly different than Longitude on Earth. Called Right Ascension in the celestial sphere, it is measured NOT in degrees, but in hours, minutes and seconds, just like a clock for good reason. Each hour of right ascension equals 15° of sky. Our 24-hour clock is measured in solar time, the time it takes for the Earth to rotate one complete time on its axis, measured from midnight to midnight. Because it is measured relative to the sun, this is a very accurate and little-changing period of 24 hours. But - if measured relative to the stars, and not the sun, the Earth keeps different time - sidereal time. This is because, in addition to spinning on its axis the Earth is ALSO revolving around the sun, one complete trip every 365 days. This results in a very slight differences between star time (sidereal) and local time (solar) of about four (4) minutes each day. In simple terms....since this is not a treatise on celestial mechanics.....a star that is EXACTLY OVERHEAD tonight at 10 p.m. local time will pass directly overhead tomorrow night at about 9:56 p.m., or four minutes earlier. In one month, that difference in sidereal vs solar time is a whopping two hours (30 days x 4 min/day = 120 minutes)! This "gain" results in different constellations and stars gradually rising earlier each successive night. Eventually, there will be different constellations seen at different seasons. But in the course of a lifetime....each season will ALWAYS show its own set of constellations! ----------------------------- Apparent Motions of the Stars Throughout the Year - The Earth's Revolution: Our "Year" - Local Mean Time vs. "Star Time" Because of the difference in sidereal time and local solar time, (resulting in all stars appearing to rise about 3 minutes 57 seconds earlier each successive night) you will note that the reference stars will change monthly as a result. This constitutes the Earth's "Year." Thus, your ideal reference stars are those identified as bright and visible, close to your desired object for that particular night you are observing. Apparent Motions of the Stars In One Night - The Earth's Rotation: Our "Day" As the Earth turns on its axis in its 24-hour day, the stars appear to rotate from east-to-west throughout the course of every night. A star rising a 9 p.m. will - at midnight - be on the meridian, an imaginary line extending from the North Star, Polaris, directly overhead and continuing to the due south horizon (this is as high as ANY star of a particular declination will reach from your location); the same star will set at approximately 3 a.m. (See Figure 3 --------------------------- A Note About Star Motions and Using Setting Circles Through the Course of the Evening - IT IS NOT ADVISABLE TO USE THE SAME REFERENCE STAR FOR SETTING CIRCLE ACCURACY THROUGHOUT THE NIGHT; as described following, always choose a reference star (particularly for setting circle and computer navigation) CLOSE TO THE OBJECT/AREA you wish to explore. This will increase your accuracy for telescope pointing via circles or computer (as well as assist you in quickly learning the many fascinating stars and constellations of the nighttime sky!). OR, you might choose to re-set the RA circle (as described following) and simply move from your present object's coordinates to those of the new object you wish to view! --------------------------- PRESENTING YOUR ETX / LX SETTING CIRCLES! POLAR ORIENTATION MODE AND THE USE OF SETTING CIRCLES By now you have undoubtedly figured out that "Alt-Azimuth" celestial alignment (one of the wonderful options with AutoStar and the ETX / LX scopes) and using setting circles DON'T MIX. You must use "Polar Mode" ("SETUP / TELESCOPE / MOUNT / POLAR") and be tracking with the fork arms tilted to your latitude toward the NCP (see my "Enhancement Guide....", Part 3 and Part 5 for celestial Polar alignment and "home position"). Regardless of if you are a dyed-in-the-wool Polar user (as am I) or a devout Alt-Azimuth activist (as most of you), you STILL must understand how about celestial motions (my crash course above) and HOW to use those mysterious setting circles. What, for example, would you DO if the batteries went dead? If you were at an observing party and the AutoStar fried? If there was a blackout in Los Angeles? Most of you would be dead-in-the-water as far as finding your ways around the sky....that's what! So at least let's study the basics of setting circle use! The telescope has TWO types of setting circles (well, "three" if you include the telescope's computer LED), one for DECLINATION and the other for RIGHT ASCENSION. Remembering that "declination," like "latitude" NEVER changes from moment to moment, the DEC CIRCLE will always be fixed, reading sky angles that do not change (unless the setting circle has inadvertently been moved or jostled from a correct position [SEE "Enhancement Guide....Kochab's Clock," Part 5 on very accurate DEC CIRCLE adjustment setting]). On the other hand the RIGHT ASCENSION (RA) CIRCLE moves its reading very slowly as the telescope moves to keep track of the objects it is centered on. It follows the TELESCOPE and not the pointer on the base! Therefore, EVERY TIME you change objects, you MUST reset your RA CIRCLE to find that object, as described following.