Watching satellites



Satellite viewing is an easy skill to pick up in amateur astronomy, and requires little or no equipment. It's also fun and does not require one to stay up late at night, since most satellites are visible only in the early evenings (or mornings, if you are inclined to get up before everyone else).


What are satellites?

The Earth has two different types of satellites: a natural satellite, our Moon; and artificial satellites. Satellites are objects which orbit a larger body, like the Moon which orbits the Earth.

Due to the rapid (read: rabid) advancement in technology in many countries, there are now countless artificial satellites orbiting our planet.


Purposes of satellites

These are various uses of satellites:

1) Astronomy

The Hubble Space Telescope (HST) and International Space Station (ISS) are just about the brightest satellites that one can see in the sky.

2) Global-positioning

The Global Positioning System (GPS) has 24 satellites in orbit currently. These help a person on earth determine where he/she is at any given moment.

3) Earth-monitoring sytems

Upper Atmosphere Research Satellite (UARS) and SeaSat 1 (monitors ocean currents and temperature) are some of the satellites that help geologists and weather-forecasters do their job.

4) Entertainment and Interest

For satellite TV and radio, for example. Also includes ham radio satellites, like the Oscar satellite series.

5) Classified information

No comments.

This list is not exhaustive. There might be some other uses of satellites that I'm not familiar with, or plain forgot about.


Best time for viewing a satellite

The best time for viewing satellites is in the wee hours of the morning before dawn, or just after the sun sets. This is because the sun is still below the horizon for the sky to be dark, yet not too far below the horizon for the satellite to catch and reflect the light back to Earth.

As satellites move pretty quickly (they orbit the Earth about 14 times a day, on average), it is important to have accurate timing as there is only a short window of time (about 10 minutes) to view them.


How bright can satellites get?

This depends on the size and "reflectivity" (ability to reflect) of the satellite. Like stars, this means that not all satellites are easily seen. A pair of binoculars or a telescope can help in viewing a faint satellite.

As far as I know, some of the brighter and more well-known satellites are the International Space Station (ISS) and the Hubble Space Telescope (HST), both of which can reach about magnitude 3 or 2 at least (quite visible with the naked eye - the brighter the object the lower the magnitude).

Of course, there are also iridium flares. This is caused by the light of the Sun being reflected by the antennae* of certain satellites. An iridium flare can reach negative magnitude, and even be brighter than the planet Venus (magnitude -4, which is *very* bright). However, they last only for a moment.


Conditions needed for viewing satellites

A clear, dark sky is ideal for viewing satellites and their flares; however, people have viewed satellites in broad daylight before. Just be careful NOT TO POINT YOUR BINOCULARS OR TELESCOPE AT THE SUN AND LOOK THROUGH IT. I strongly recommend naked eye viewing in the daytime.


Equipment needed for viewing satellites

A whole sky map comes in handy to identify the constellations.

A compass is needed if you're not sure if the direction you're facing is the right one. If you need to determine your coordinates precisely, a handheld GPS system is needed.

A watch is essential as satellites wait for no man. Tardiness means that you might not get to see your satellite.

Binoculars are useful to zoom in on the satellite, especially if its just a faint speck (the sky may not be completely dark, so it might be difficult to view them with eyes alone). Telescopes are optional. The trouble with a telescope is that if you're not familiar with it, and have to fiddle about a lot with it, the satellite might just disappear from sight before you can view it properly. But if you're expecting an iridium flare, please TRY TO VIEW IT WITH YOUR NAKED EYES. Otherwise you might suffer from temporary blindness after viewing an extremely bright flash of light in a dark environment, when your pupils are dilated.

Lastly, you need to know the time that a satellite will pass by your location, and the direction in which it would appear. As satellites move much more quickly than stars, there should be no problem distinguishing them from stars if you are stationed at the site before it appears, and watch carefully for any "abnormal star" that appears. You can get information about satellite passes from http://www.heavens-above.com/ . More is covered in the next section.


Information needed for viewing satellites

1) Know thyself...

Coordinates are needed to know when you can expect to see a satellite. For a general view of a satellite, Heavens Above (http://www.heavens-above.com/) recommends coordinates within a 10 km radius. For iridium flare viewing, coordinates within +/- 1 km are best.

2) And know thy satellite...

The direction from which the satellite is approaching, the duration of the pass, and the magnitude of the satellite will help in its identification. Otherwise you might mistake it for just another star... until you realise that its going past too fast. Oops! Bye!



If you're interested in finding out more about satellite viewing, I recommend the following sites:

Heavens Above! - http://www.heavens-above.com/
This site is great! It has a large database for coordinates all around the world, even a small country like Singapore has several places listed.

Iridium Flares - http://www.satobs.org/iridium.html
About how and why iridium flares occur.

Google - http://www.google.com
This looks dumb, but everything I learnt about astronomy (especially the minor branches of it), I learnt from Google searches.



*doyle says: Iridium flares are caused by reflection off a teflon-coated aluminum panel of an Iridium satellite--Iridium is the name of the satellite, not the material it is made from. The flare can last 5 to 20 seconds--really impressive if you've seen one.

doyle says:One final nitpicking--if you have a star map, a compass seems superfluous, unless you are just starting to learn the sky.

Thanks also to wertperch and lj, who corrected me when I wrote that satellites orbit the Earth "14x times on average". The satellites orbit 14x a day on average, not just 14x.

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