Caoimhín's Content

We are coming up to the 25th of December, Christmas for many people, and so that is the date used mostly in this video, but the topic of the video is satellites. Satellites can be surprisingly easy to spot once you know how, and although they are artificial, unlike most subjects of my videos, they are still an interesting group of objects to observe.

Sunset and sunrise are the best times to look for satellites. They don’t really make any light of their own, at least none that matter for observing them. Instead, just like the planets and the Moon, we see them thanks to sunlight that they are reflecting. They are so much closer to us than the Moon that when they are directly behind the Earth they are usually in our shadow, so it is almost impossible to see them in the middle of the night. At sunset and sunrise, their solar panels are more likely to catch the Sun’s rays at the right angle to reflect them down on us, making them nice and bright.

To prove that point, starting on the 22nd, in the city with all the light pollution that comes with it, during sunset with the orange glow it creates and at a time of the month where the Moon is up and close to full, you can see a satellite with your naked eye. The ERS-2 satellite pops up in the south-west, almost under Jupiter, and then zips up next to Saturn, over the Summer Triangle and then down into the north-west. This should happen at around 5:30, so nice and early in the evening. The ERS-2 is an old research satellite that is no longer in use and eventually it will burn up in the Earth’s atmosphere.

The ERS-2 isn’t particularly big, where as the next target is huge. We move into morning time and into the 23rd to see the ISS pop up in the sky above Venus at about 5:55. The International Space Station is massive, with big reflective solar panels, making it a very bright object when it catches the sun’s light, and essentially invisible otherwise. This allows it to seemingly appear in the middle of the sky before curving down to set, or sometimes to vanish high in the sky. This is only a trick of the light, you don’t need to worry if the ISS seems to vanish or appear, it’s just moving in or out of our shadow.

The ISS is a pretty low orbiting satellite, it is in Low Earth Orbit. Satellites in this orbit move across the sky quickly, so this can make them a little trickier to spot. They will usually be in the sky for a minute or so at least, so as long as you know where to look you can catch them. When the ISS comes back around at a bit after 7 o’clock in the morning, it’s visible for much much longer. That’s right, the ISS gets the whole way around in just a couple of hours, it averages 93 minutes per orbit. When we see the ISS closer to sunrise, we get to see it travelling across a much larger par of the sky, practically from horizon to horizon. Although the ISS orbits the Earth about 15 times a day, its orbit shifts around the planet, so you aren’t guaranteed to see it every night. Here however, we have a run of a few days, the morning of the 23rd, 25th and 27th, with the ISS visible twice, at around 5:50 and again at around 7:30.

At least, that seems to be the case right now. The ISS, and almost all satellites, can change their path if they need to. This is especially true for satellites built to observe the Earth, so they can look at different places. This means that predicting them into the future is a little harder. The paths of the planets don’t really change, so we can make predictions years and years into the future, where as predicting the ISS more than a month into the future can be unreliable. Usually, the ISS has its maneuvers planned in advance, usually weeks, so predicting a week or two into the future is usually fine. Here, we’re barely looking a week ahead so we should be fine, but if you miss the ISS and want to look for it next year, you’ll need to double check its position. Luckily NASA and ESA track the ISS and make its position publicly known, so it isn’t too hard to get updates.

Moving through a dark sky at the right time, we can see that there are loads of satellites zipping around. With time sped up in Stellarium, they really seem to whizz around. They don’t move that fast, but they are appreciably moving across the sky in real time at real speed as well. This is more true for the Low Earth Orbit satellites, satellites further out orbit slower or even not at all. A satellite at the right height above the Earth will stay pointing at the same spot, making it a geostationary satellite. These are always much further away as well, which doesn’t help in observing them.

While the smaller satellites are tricky to spot in the city, the ISS is certainly bright enough to be visible, as are many other satellites and man made objects like rocket stages. There are always plenty of opportunities to see the ISS, it regularly passes over almost every part of the Earth as its orbit drifts around. Being visible early in the morning here in Ireland, on a date where many people will be waking up early to eagerly open presents, is a particularly nice coincidence. It’s especially handy that it happens a few days in a row, so if you miss it on the 25th you can try again on the 27th, hopefully with a new telescope or set of binoculars. If you do take a closer look, try it at 5:55 rather than 7:30, just to keep your eyes safe.

That brings us to the end and almost the end of the year. The upcoming videos are going to start looking ahead to New Year’s Day and beyond, so I hope you’ll stick around to get a preview of what’s to come.