Time: 2:07(.59) AM
Brief
Yesterday, I glazed over the transit of the Summer Triangle; listing the time and showing it. However, it is not obvious why the Triangle's transit time is what it is, given that none of the stars are at transit at the same time. Altair is close, yet not exactly the same as the "center" of the triangle.
To find the center, we have to think geometrically; remember that in high school or college?? ;-) Oddly enough, after using the tools on the software to find out where to find the center of the Triangle, it helped me remember a little from those days. Before getting into the detail, here is one more showing of it at transit with both the alt/az grid and meridian included. I marked zenith also to show the relative positioning of each star.
Detailed
First of all to quickly clarify, the time above shows the transit time of the Triangle's center. Now, what do we do to identify that?
*If a perfectly straight line is drawn from each of the Triangle's stars, regardless of whether it or any other triangle is equilateral, isosceles or any other shape, that same line when bisecting the side opposite leads to all three lines meeting at a common point.*
When setting the Triangle at a transit time one evening, I tried this, and it was true, meaning that the software engineers did well! That is really all there is too it. With other asterisms not so simply geometric (the Big Dipper for example) it may be much more complicated than that to identify its center. We will look more into other asterisms' centers some other time. Using the Summer Triangle is to simply point out that all constellations, asterisms, deep-sky objects, etc. all have transit times. For most of them, it is when they reach their maximum altitude, as the case of the Summer Triangle's center. As for the stars of the Triangle during the center's transit, Vega does so about 1 hour, 5 minutes earlier; Deneb almost exactly 1 hour later (a little over 59 minutes); Altair just a little less than 9 minutes later. Use the grid in the image as reference for each, with the altitude ones circling outward from zenith.
There is another way to roughly identify the center, now that we know the transit times of each star before and after the center's, seen from mid-northern latitudes: using Altair and seeing it about as high as it will go, we have a good idea of when the center transits even if we were not to know the geometry procedure mentioned earlier. At different latitudes of course, the transit time differences of each will be different, and the Triangle will not even be seen the same when looking directly south or north, as far as which star is considered the apex; that is alright though. Spend the next several weeks enjoying the star high up, and for the rest of this month, seeing it high enough above the horizon all night along. Albeit it a short night being summer for our hemisphere, it is still a decent number of hours!
As a final note, the altitude of the center of the Triangle at transit is 83.9º as seen from out exact latitude. Therefore, using the image above, look at where the 85º altitude marker is, between the r for summer and T for triangle. At the tip of the r's "stick-out" and slightly below the hard-to-see 84º marker, is the 83.9º mark the center, almost exactly. Neat?
This second image is a zoom-in to show it a little more clearly, and the Triangle filling up the whole field. Try this with your telescope sometime, if you have an alt-az one carefully marked and calibrated (be level to the ground too) and tell your astronomy buddies that you are looking at the center of the Triangle! Of course, once you find it, chart the star field so it is a little easier to identify in the future.
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