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Treebeard's Stumper Answer
30 March 2001

Catch an ISS Pass

We've been reading science fiction in my science classes lately, but NASA has been making it science fact with the International Space Station (ISS). At just the right time and place, the big ISS is visible as one of the brightest and fastest objects in the sky. We have several opportunities to view it next week. (See below.) The challenge is to observe an ISS pass. Early fog and sunrise and city lights might be a problem. The stumper is why all the view times are in the early morning and evening. Wouldn't it be easier to catch an ISS pass late at night when the sky is darkest?

View of the International Space Station (ISS) from the approaching Space Shuttle Atlantis on February 28, 2001. Space shuttle astronauts on this mission conducted the 100th space walk by an American. That's the Rio Negro region of Argentina two hundred kilometers below.

Notes: Here's a table of the next few visible International Space Station (ISS) passes over Dunn Middle School in Los Olivos, California (34.6680°N, 120.1140°W, Epoch 1 April 2001). Pass times from Santa Barbara to Santa Maria are just seconds different. This data is from the Heavens-Above site. The predicted viewing times change slightly from day to day because of course adjustments and other variables, but you can check the site for updated info and detailed sky charts. You have to specify your location, but it's easy to select from their huge database. (Or just click here for Los Olivos, Santa Barbara, or Santa Maria.)

These times are all Daylight Savings Time (PDT). Because of our usual morning fog, the best time to catch an ISS pass here in central California looks like next Wednesday evening, April 4, starting at 8:46 PM (PDT). There are many other viewing opportunities this coming week, morning and evening.


Where the ISS is now, moving to the right.
Reload this page a few times to see it move!
(Image from Heavens-Above.)

All times are Pacific Daylight Time (PDT). Magnitude (Mag.) is brightness. The smaller the number the brighter the ISS appears, and negative numbers are brightest of all. Altitude (Alt.) is how high to look in the sky, measured in degrees with 90° straight up. Azimuth (Az.) is the compass direction around your horizon. Note that an entire pass only lasts a few minutes, so don't be late!

Date Mag. Starts Max. Altitude Ends
Time Alt. Az. Time Alt. Az. Time Alt. Az.
Sunday, April 1, 2001 -0.5 05:38:59 PDT
5:39 AM
12 NW 05:41:51 74 NE 05:44:58 10 SE
Monday, April 2, 2001 1.6 04:42:23 PDT
4:42 AM
26 ENE 04:42:23 27 ENE 04:44:34 10 E
Monday, April 2, 2001 0.9 06:15:16 PDT
6:15 AM
10 W 06:17:38 20 SW 06:19:58 10 S
Tuesday, April 3, 2001 -0.2 05:18:36 PDT
5:18 AM
45 S 05:18:36 45 S 05:21:04 10 SSE
Tuesday, April 3, 2001 0.9 20:11:48 PDT
 8:11 PM
10 S 20:14:01 18 SE 20:14:25 18 ESE
Wednesday, April 4, 2001 -0.7 20:46:29 PDT
 8:46 PM
10 SW 20:49:36 81 NW 20:50:35 40 NE
Thursday, April 5, 2001 0.1 19:46:29 PDT
 7:46 PM
10 SSW 19:49:19 33 SE 19:52:10 10 ENE
Thursday, April 5, 2001 2.0 21:22:57 PDT
9:23 PM
10 W 21:25:18 19 NNW 21:26:08 17 N
Friday, April 6, 2001 0.6 20:21:35 PDT
8:22 PM
10 WSW 20:24:33 42 NW 20:27:32 10 NE
Saturday, April 7, 2001 2.6 20:58:19 PDT
8:58 PM
10 WNW 20:59:54 13 NNW 21:01:30 10 N
Sunday, April 8, 2001 1.6 19:55:58 PDT
7:56 PM
10 W 19:58:37 25 NW 20:01:17 10 NNE

Here's two more ISS stumpers I've been thinking about:


The weather did not cooperate with my attempts to view the International Space Station this week, but there will be other times. The ISS orbits the Earth 16 times a day! But satellites are only visible in the early morning and evening when they are lit by the sun while it's dark for the observer. Think of camping at the base of a tall mountain. The sun sets and your camp is in shadow, but the peak still catches the light for a while as the sun sinks lower and the darkness moves up the summit. Late at night, the summit and the satellite far above are both in the dark.

Notes:

I first understood Earth-satellites many years ago while camped by Desolation Lake in the high wilderness basin just west of Mount Humphreys in the High Sierra Nevada mountains. The sun set early at my camp, but that great peak caught the setting sun in alpenglow for a while until the sun set for the summit too.

Now imagine a mountain 200 km high! As the sun sets lower and lower, you will see the shadow line climb up until only the very summit is lit, then it too is dark. Now take away the mountain except for the very top, and that's the satellite! That's why we can usually only view Earth satellites soon after sunset and just before dawn. They're close enough to Earth to be caught in its shadow when the sun is too far below the horizen to light them, so satellites are only visible for a few hours after sunset or before sunrise. The exception is during summer at high latitudes far north or south where the sun is never too far down, and some satellites can be seen the whole night through if they pass over in polar orbits. This also explains why even very bright satellites often fade away as they move across the sky and enter the Earth's shadow.


Another view of the ISS now from www.space.com.

The real stumper for me is to understand the ISS orbit. The Space Station has been potentially visible (through the clouds) every day for the last 10 days here in Santa Barbara, but we won't get another chance until at least April 18. Why are viewing days clumped in groups? Why does the time change from morning to evening? Why does the direction change from north to south? Here are predicted viewing times for Santa Barbara over the next 90 days, as figured by Scott Hather's freeware SatScape program. This table only shows visible passes with altitude > 10° high. The data will soon be out of date as the orbital elements changes. June 2 looks remarkable with 3 visible passes. We'll be camped at our usual place at the Live Oak Music Festival for the June 15-17 views!

DateTimeStart Az.Alt.DurationEnd Az.

18 APR 200121:06:22005 1200:02:23050
19 APR 200121:38:51330 3300:05:46106
20 APR 200120:37:22350 1600:04:04072
21 APR 200121:10:00321 5500:06:07121
22 APR 200121:43:33288 2500:05:17171
23 APR 200120:40:33312 7400:06:11136
24 APR 200121:14:17273 1600:04:01191
 
29 MAY 200104:48:53010 1100:01:46046
31 MAY 200104:03:10008 1100:02:05050
01 JUN 200104:26:18333 2900:05:11101
01 JUN 200122:26:28296 1300:03:11003
02 JUN 200103:16:30006 1200:02:13052
02 JUN 200104:50:12307 5000:05:43143
02 JUN 200121:13:54251 3500:05:26031
03 JUN 200121:38:58296 1300:03:06002
 
14 JUN 200122:34:01343 2000:04:24086
15 JUN 200122:53:41318 5900:05:35126
16 JUN 200121:39:17347 1700:04:02079
17 JUN 200121:58:14322 4400:05:29119
18 JUN 200122:17:32296 3300:05:10159
19 JUN 200121:01:35327 3600:05:19112
20 JUN 200121:20:07302 4400:05:23150
21 JUN 200121:39:26263 1300:02:50201

I think this is a strobe light effect, like how wheels seem to go backwards in film. The ISS is in a regular orbit around the Earth every 90 minutes or so. 90 minutes is exactly one-sixteenth of a day, so the Earth rotating underneath in 24 hours should bring the satellite back to almost the same place in the sky after 16 (24 / 1.5) orbits. Almost, but not quite. Playing with simulator software, it appears that the ISS orbit is a bit faster than 90 minutes, so its path seems to regress a bit westward with each orbit. In time, it moves out of that narrow window where it is in the sun and we observers are in shadow, so we can't see it again until it has regressed a full orbit. That's why viewing opportunities come in clusters. It has it's own orbital frequency, but we actually see it at a different frequency that also depends on the sun and our exact location on the Earth.

Visualizing satellite orbits in 3D is tricky. Here's an image of the April 4, 2001 pass over Santa Barbara, made with WxTrack. It shows several successive passes, so you can see how the orbit shifts to the left/west with each pass. In time, it will pass over most of the Earth.


WxTrack image (modified), April 4, 2001 at 8:46 P.M., PDT.

The sine wave-like orbit looks complicated, but note that the day-night line has a similar up-and-down shape. In fact the ISS is orbiting around the center of the Earth in an almost-circular orbit. As the Earth turns underneath, it creates this elegant curve. The ISS orbit was deliberate. The International Space Station must be accessible from all participating nations. The not-quite-polar ISS orbit swings north and south at an inclination of 51.6 degrees from the equator. This brings the path over both Cape Canaveral and the Russian Baikonur Cosmodrome in Kazakhstan (lat. 48° N) for easy access.

I tried to catch the ISS pass on the last day of our viewing cluster. It wasn't a great pass with magnitude 1.8 and a max altitude of 24 degrees. The best view was just after 8:00 pm, and it wasn't quite dark yet, and I didn't see anything. *Sigh* There will be other times. But I did log the images from Heavens-Above every 10 mnutes, and I stitched them into this animation.

Note how far the ISS moves every ten minutes! This animation shows two passes over Santa Barbnara on the west coast of the US, but only the first was potentially visible. Note how the ISS orbit always looks like a straight line, though the Earth keeps turning below. It would be better to keep the ISS orbit stationary and show the Earth turn beneath it.

Here are some links for further research:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org