ISStrackJul19- by John Hlynialuk, Bluewater Astronomical Society

Mark your calendar for 11 pm on the night of July 19. The Sun sets around 9 pm EDT, there is a nice first quarter Moon high in the sky and you will see Jupiter shining brightly to the left of the Moon. Also in the western sky there is Venus, even brighter than Jupiter, but for the purposes of this column, we will ignore both Jupiter and Venus.

The fun stuff starts about 11:10 pm EDT and involves the Moon and the most expensive, largest spacecraft ever built (with 6 humans aboard) called the International Space Station, or ISS. The event we will see (weather permitting) is a transit of the ISS across the face of the Moon. You can't miss it, because on July 19, as it crosses the sky, the ISS will be as bright as the planet Venus!July19ISS

You may have been lucky enough to have seen an airplane cross the face of the Moon, and though the ISS is a very small object at a distance of 500 km, it shines by reflected sunlight and can easily be spotted with the naked eye (and even followed with binoculars). ISS circles the Earth 15 times in 24 hours, and periodically, by chance, it passes in front of the Sun or Moon. In July, for example, there are 7 solar and 6 lunar transits. Solar transits require special filters to view the Sun safely and without them, solar transits are not safe to watch. However, lunar transits can be seen with the naked eye because the ISS is often visible as a very bright satellite moving across the sky like dozens of others. In fact, on July 19 alone, there are 6 visible passes of ISS across the sky during dark hours. One of them, the pass from 11:10 pm to 11:20 pm EDT crosses the Moon.

To succeed in viewing this lunar transit, find a flat horizon to the west and start watching for the ISS around 11:10 pm: -look below the Moon and watch for a point of light moving upwards toward it. You should be able to see ISS by 11:12 pm and it just gets brighter from then on, (eventually when directly overhead it will be as bright as Venus!). For 2.8 seconds just after 11:13 EDT, ISS will pass right across the Moon from bottom to top. The key is to spot it before it crosses the Moon and follow its path as it moves upwards. The ISS does not get much brighter than this and it should be relatively easy to spot early in its pass. The entire pass from horizon to horizon takes over 10 minutes.

There is only one catch. To see ISS cross the Moon, you have to be in the right spot on the planet. The path is about 10 km wide with the centre line running from just north of Sauble Beach to White Cloud Island on the Georgian Bay side. The diagram here shows the path of visibility (courtesy of Anywhere on the centre line you will see the ISS cross the centre of the Moon. If you are a km or two either side, that will shift the path across the Moon toward the edge, and if you are outside the blue track shown, ISS will miss the Moon entirely. Even a miss is a neat experience especially since the space station is so bright, but being in the "shadow" of ISS on the Earth is like a kind of eclipse, only this is the space station that is eclipsing a more distant body. It is not a total eclipse because ISS is too far away to cover but a tiny fraction of the Moon's face.

The west end of the July 19 track falls across the Sauble Falls Parkway so anywhere with not too many trees to the west would be suitable. Wiarton would also work and anywhere along Grey Rd 1 from Wiarton to Big Bay would also be a good viewing location.

The track may change slightly as the time approaches so double check your information on You absolutely need up-to-date information. There are also several good smartphone apps that do the job as well. There will be other chances to see transits but start with lunar transits before you tackle the solar ones. Safe solar filters are absolutely required for those, but lunar transits are totally safe.

Good luck with your ISS-spotting!




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