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From: Claudio Silvestri, of Toronto, Ontario
To: Wayne Hale, flight director

Question: Hello, I was looking at potential ISS sighting data by city and noticed that in Toronto on Dec. 2, there would be two sightings roughly 1.5 hours apart. What are the factors involved that are allowing two sightings over the same area in such a short period of time: ISS Monday, Dec. 02/05:28 p.m. and ISS Monday Dec. 02/07:04 p.m.

Answer: Claudio, ISS sighting opportunities arise whenever your ground-based location is in the vicinity of sunrise or sunset as seen by ISS. Because ISS orbits at an altitude of about 390 kilometers, it can remain lit well after sunset or well before sunrise on the ground. You then see ISS as a bright "star" against a dark sky, weather permitting. In early December, Toronto has a sufficiently dark evening sky to see ISS if it flies over after about 5:10 p.m. If it's much after 7:10 p.m., however, ISS will have already entered Earth's shadow by the time it reaches Toronto.

For about the next week or so, ISS will see sunsets near the northern end of each orbit at no more than 51 degrees north latitude. This is very near Toronto's latitude, 44 degrees north. During this period, two types of sightings are possible. If ISS flies over Toronto near the middle of the 5:10 to 7:10 p.m. window, only one sighting is possible. In contrast, if a sighting occurs near the start of this window, another can occur 1.5 hours later on the next orbit before the window closes.

Looking at Dec. 2's sighting opportunities, the latter condition does indeed apply. On Dec. 3, the only opportunity is at 6:10 p.m. Previous and subsequent orbits on Dec. 3 fly over Toronto at 4:40 p.m. (too early) and 7:40 p.m. (too late) and are invisible. Incidentally, Dec. 4 also favors Toronto with two ISS sightings. The first occurs at 5:15 p.m., only 34 min after sunset at 4:41 p.m. Hope your weather cooperates!

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Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 12/11/2002
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