Monday, August 23, 2010

ISS Passes

Want to see the International Space Station (ISS) passing overhead this week?  It's well situated for viewing here in the Northeast after sunset and quite bright.  The reason we can see it is because it has a lot of flat metal surfaces which reflect sunlight.  While it's dark here on the ground, it's 20 miles up and can still reflect sunlight from the Sun which is below our horizon.

Here's a data table of pass times for Kingston, NY in the heart of the Hudson Valley.  If you want this data for your town, visit and select your location.

How do your read this?  Well, the first column is obviously the date and the Time colums are the local time in EDT (24-hour format).  Starts is when the ISS first appears in the sky, Max. altitude is when it hits its maximum altitude, and Ends is when it disappears.

The second column, Mag, is the magnitude or how bright the object appears.  It's a hold over for the ancient Greek astronomer Hipparchus who said the brightest stars had a magnitude of 1 and less bright stars had magnitudes of 2, 3, 4, 5, or 6 with 6 being the dimmest he could barely see.  Today, this logarithmic scale is defined a bit more precisely, and includes negative numbers, with the brightest star in the sky, Sirius, being a magnitude -1.5.  Bottom line is that lower numbers refer to brighter objects. For example, the ISS will be brightest on the first pass of August 26 with a magnitude of -3.5.  The dimmest pass will be the second one on August 27 with a magnitude of -0.1 (still brighter than all the stars of the Big Dipper).

Alt. and Az. refer to the position of the ISS in the sky at the indicated times.  Altitude is the angular height above the horizon.  An object directly on the horizon has an altitude of 0°.  An object directly over your head (at the zenith) has an altitude of 90°.  There's an easy way to estimate altitude.  Hold your fist out at arm's length and it covers about 10° of the sky.

Azimuth is simply, in this case, the compass direction where the ISS will appear.  For example WSW is half-way between west and southwest.  If you know how to find the North Star, it's easy to figure out where to look (I should post on this sometime).

So, if it's clear Thursday night, for example, go outside around twenty after eight and look for a very bright light moving across the sky from the SW horizon, to half-way up the sky in the SE, and then disappearing in the ENE.

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