Perspective on portion of the sky covered by pictures

waljohnson · Post by **waljohnson** » Fri Jun 30, 2006 5:43 am

Sorry for the banal newbie post, but I have young children who I am trying to interest in Astronomy and science in general. They love the pictures, but it is usually very difficult to put them in perspective. For example, today's APOD of "The Antennae" in Corvus. It is, as usual, an amazing picture. But I'd like to be able to show the kids how much (or little) of the sky it takes up. In this case, I assume arc-seconds. But is there a way to have a larger view of the sky and say, "look, this photograph is of this little square here"?
Again, I apologize for the naivety, but I think it would help my kids learn a lot....

Thanks

Qev · Post by **Qev** » Sat Jul 01, 2006 1:06 am

Well, the field of view in this image is roughly 30 arc-minutes, or half a degree on the sky. This is almost exactly the diameter of the full Moon (or the Sun) on the sky, interestingly enough, which also makes for a convenient measuring stick.

Sometimes they don't give the actual angular size of images, but instead the distance and estimated size of the object. With a bit of trig, though, you can convert back to degrees-minutes-seconds.

waljohnson · Post by **waljohnson** » Sat Jul 01, 2006 3:34 am

Thanks.
Not being an astronomer I've never done trig on this scale. So 30 arc-minutes is half a degree, and at 60 Million light years, that arc would span about half a million light years. Which makes this structure rather large. Going back and looking at the APOD I see that it tells me all of this, but knowing that the moon is about 30 arc-minutes puts it in perspective. Thinking of the size of the area that arc covers 60 million light years out there is, well, quite incomprehensible. I'm not sure my six year old will get it!

Thanks, I learned something!

Qev · Post by **Qev** » Sat Jul 01, 2006 4:24 am

Happy to help! It's actually a very straightforward calculation, as long as you have a calculator that'll do the inverse tan function (Window's built in calculator has it in scientific mode). Basically it's just:

angle = tan-1 ( size / distance )

I just found a handy tool for calculating this, also:

http://www.1728.com/angsize.htm

It never really ceases to startle me just how large these objects actually appear on our sky; I always thought things like distant galaxies and the like would be tiny to the point of being invisible. But these are huge structures. A good example is the Andromeda galaxy; it looks like a tiny little smudge to the naked eye, but it actually extends several times the Moon's diameter across the sky, it's simply so dim that most of it is lost to human vision.