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In my area, from December 17th - 26th, you can see the space station every day, twice

Screenshot 1

But from December 27th - January 6th, you can only see it twice

Screenshot 2

My question is, why does it appear for almost the whole month of December, and then not appear for a few days? Is it based on my location? I live in Alberta, Canada if that helps.

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3 Answers 3

up vote 11 down vote accepted

Two basic conditions need to be met for anyone on the globe to observe the International Space Station (ISS) from any location:

  • ISS needs to pass roughly overhead of your location, and
  • it needs to do that during night so it's visible to the naked eye

Now, obviously there are other requirements, like e.g. weather (if it's overcast we won't be able to see it through the clouds), but those screens you attach don't account for that, since it would be nigh impossible to predict weather so far in advance.

Let's discuss the two basic conditions mentioned a bit more. ISS has an orbital inclination of 51.65°. Why is that is explained in this answer on Space.SE, but to quickly recap, this orbital inclination was chosen because it's near ideal for launches from the Baikonur Cosmodrome, where most launches towards the ISS are made from, it is still achievable from the US launch facilities and most US built launchers, and it allows the ISS to go over majority of the populated areas on Earth. So it was decided both for economic reasons, as well as to allow ISS to do as much science as possible.

But what this inclination means is that the ISS will make a ground track (path along the surface of the Earth directly below it) that moves towards the West with each new orbit, even though the ISS itself moves West to East (or saying it otherwise, it's in a prograde Low-Earth orbit). Its ground track could look something like this:

              enter image description here

                  Ground track of the International Space Station with an area where it is observable from the Earth's surface
                  shown in the white circle (Image source: ISS Tracker)

ISS orbital period, the time it takes it to circumnavigate the globe and complete one orbit, is roughly 93 minutes. As you can see in the screen grab from the ISS Tracker above, its ground track moves westward on each new orbit, which is due to Earth's rotation on its axis, so this shift towards the West will be equal to how much the Earth rotates in the meantime, which in those 93 minutes comes out at roughly 22,5° or a good 2,505 km (1,556 mi) of the Earth's total equatorial circumference.

But ISS's orbit isn't Sun-synchronous, meaning it won't be over any given Earth latitude at the same local mean solar time, so when it will be observable depends also on its orbital position relative to the position of the Sun. We require that it is dark, remember? ISS isn't bright enough (doesn't reflect enough light with its truss and solar panels) during the day to be brighter than the day skies are. So we have another parameter, which is local time. As you probably anticipated by now, the length of the day isn't really equal at any day of the year, and moving further away from the equator only increases this difference of the length of the day in winter, or the length of it during summer. This is another range or values that your software you're attaching print screens from has to account for.

So, when your software collects data from you regarding your geolocation and current date and time (might do that automatically if you're using it on a device that has GPS enabled), it will establish the times (timetable) of when the ISS ground track is over or around your area, and if that happens during night. The ISS doesn't have to be exactly overhead, at zenith, so it will also calculate angle to the zenith for any area that the ISS goes over, at which you would still be able to see it.

Understanding this, even if we don't look at your timetable, we could establish that if you're at latitude to the equator that enables you to see the ISS, you'd be able to do that roughly a few times in a row separated by around 93 minutes, then wait for the ISS ground track to come back towards your longitude (16 orbits or about one day and an hour), and do that during night when you'd be able to observe it. Nights are long now during December at Northern latitudes, so you see a long row of couples of such chances during each night. As long as the weather holds, of course.

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Best viewing will be shortly before sunrise or shortly after sunset so your sky is dark, but not so late that everything above you will be in the Earth's shadow.

It will be brightest when it's near the opposite horizon from the sunrise/sunset, because the solar cells will be perpendicular to the sun, and nearly perpendicular to you.

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In addition to the requirements that it be night, and the ISS be overhead, the space station must also not be eclipsed by the earth's shadow. With earth taking up roughly 88 degrees of sky from the ISS's typical altitude of 420 km, eclipses correspond to night-time on the space station. They're each 45 minutes long, and happen 16 times a day. Harmonics between earth day/night cycle and ISS day/night cycle account for those mysterious gaps in visibility noted by aman207. The start of such eclipses are oten observed from the ground: You'll see the ISS come over the horizon like a ball of fire, get halfway across the sky, then fade to nothing in a minute or two.

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