How many arc seconds should the ISS appear when it is 400 miles overhead? Compared to diameter of the moon?
I swear I’ve seen its multiple modules and the solar panels at 45x but the Space Station was traveling across the field of view so quickly, I wasn’t sure of what I saw. Can it big as big in the eyepiece as I imagined?
The ISS main truss is almost 300 feet long. That’s 0.0568 miles (about 1/20th of a mile). The tangent of the angle is 0.0568 / 400, or 1.4205e-4.
To convert to angle, use the arctangent function, and get 0.00814 degrees. That’s equivalent to 0.4883 minutes, or 29.299 seconds.
Typical eyepiece (Plossl) has about a 50 degree apparent field of view, and if it yields 45 magnifications, that is about 1.1 degrees true field of view. The moon’s diameter (about 30 minutes) would about half fill the view. The ISS would appear to be 0.4883 x 45 = 21.97 minutes wide in the eyepiece, or about 2/3 the size of the moon TO THE NAKED EYE. Actually, that’s pretty big, so maybe yes, depending on your imagination.
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The ISS main truss is almost 300 feet long. That’s 0.0568 miles (about 1/20th of a mile). The tangent of the angle is 0.0568 / 400, or 1.4205e-4.
To convert to angle, use the arctangent function, and get 0.00814 degrees. That’s equivalent to 0.4883 minutes, or 29.299 seconds.
Typical eyepiece (Plossl) has about a 50 degree apparent field of view, and if it yields 45 magnifications, that is about 1.1 degrees true field of view. The moon’s diameter (about 30 minutes) would about half fill the view. The ISS would appear to be 0.4883 x 45 = 21.97 minutes wide in the eyepiece, or about 2/3 the size of the moon TO THE NAKED EYE. Actually, that’s pretty big, so maybe yes, depending on your imagination.
References :
Proportions, trigonometry, and stuff like that
http://www.perseus.gr/Astro-Sat-Trans-2006-04-14.htm
The Moon’s apparent diameter is a bit more than 30′ (arc-minutes)
The ISS’s apparent diameter (using the first posting) is a bit less than 30" (arc-seconds).
That is 1/60.
However, the 30" is probably a maximum — when ISS is directly overhead. If you see ISS away from the zenith, it is that much further away from you (and, therefore, smaller). However, the Moon’s distance would be (relatively speaking) the same
– still 30′.
The web site above shows multiple images of the ISS as it transits the Moon, as seen from Earth. It is the series of black dots near the bottom. Compare it to some of the smaller craters.
You can still (barely) make out some details (solar panels), but 45x would be stretching it.
There are web site that predict Moon transits of ISS. Find a suitable transit and fix your telescope on the Moon (much easier to track).
Here is a clear view from another web page:
http://pictures.ed-morana.com/ISSTransits/
Another nice sequence by Ed:
http://antwrp.gsfc.nasa.gov/apod/ap061014.html
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I’ve looked at it in 10x binoculars and it definitely doesn’t look like a point. I was able to make out a vague ‘T’ shape. It’s very bright and the glare makes it difficult to spot detail, so I’ve been meaning to try when it comes over in twilight. When overhead, it is closer than 400 miles. It’s orbital height is about 350 km (217 miles). If its width is 100 metres, its angular diameter when overhead would be about 1 minute of arc. Even in the 10x binoculars, it should appear 1/3rd the diameter of the Moon.
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Well, it is only about 220 miles up not 400. It will look fairly large in a telescope, but i don’t know the exact size in arc.
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http://www.flickr.com/photos/jrlasers/2881051470/
between 30 seconds to a minute of arc time depending on the weather.
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