Monday, January 1, 2018

International Space Station Transit

Happy New Year and welcome 2018!  There is much to celebrate!!

It is fitting that 2018 started off for me with an astronomical adventure, as it provided some fodder for a blog post...and in addition, New Years Day is the anniversary of this blog!  It all began here, in Googleland, on January 1, 2010.  While I have trailed off in the frequency of blog posts, my enthusiasm for astronomy remains strong.  The icing on the cake, is that today is the 22nd anniversary of my marriage to the love of my life, Beth.

As for the astronomical adventure, it happened that there was a transit of the International Space Station across the disc of the Sun as seen from central Tucson today.  This is the third time I have observed a transit of the ISS across the Sun, and you can read about the first adventure here, and the second here.

Today, I headed out to Himmel Park in central Tucson with my friends Jerry and Travis and set up my TEC 140 refractor with a Herschel Prism.  At the last minute I decided to use my Canon 6D to record video, instead of attempting to 'burst shoot' the transit.  This would take a lot of stress out of having to touch the camera and capturing an event that lasts approximately 1 second!

Without writing a novel, it was a success!  I am posting the original video at the bottom of this post, and I also created a composite of 32 individual frames showing the transit.  You can see in the composite image below that we were not perfectly centered under the ISS shadow, but we were very, very close.

Click the image to enlarge

To appreciate the scale here, consider that the Sun is about 93 million miles away and the ISS was approximately 421.5 miles downrange from me during the transit!  Because the ISS is so close, one must be on a very narrow ground track of the ISS' shadow in order to have the transit appear centered on the Sun.  Today's transit lasted 1.4 seconds on the center-line.

How fast does the ISS orbit?  It orbits the Earth in approximately 92.6 minutes, which results in an apparent ground speed of 7.885 km/second (4.9 miles/second)!  The ISS is also fairly small-  It's dimensions are approximately:
Length 72.8 m (239 ft)
Width 108.5 m (356 ft)
Height ≈ 20 m (66 ft)

The distance of the Space Station today, coupled with it's actual size, translates into an apparent angular diameter of 41 arcseconds- and the Sun is about 1800 arcseconds in diameter.  So the ISS is about 2.2% of the apparent diameter of the other words, you could fit about 44 ISS's across the Sun's face today.  You can now appreciate why a video capture is the easiest.

Here is the short close attention as the transit is only 1 second!  Watch it full screen for easier viewing.

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