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info on ISS visibility
On this page I will try to explain satellite (or better: ISS) visibility.
I do this from my own views and experience.
What is needed to see ISS?
First of all, ISS must actually be above the horizon. This is the easy and logical part.
ISS is a huge structure, and the reason we can view it from the Earth's surface is because the SUN
shines it's light on it. However, against a daylight sky, our eyes will never catch it. So it needs to be dark.
Here is another problem: ISS is only illuminated by the sun, when it can 'see' the sun, so when the sun
is not behind the earth.....
In general this means that the sun cannot be too far below the horizon. Or better: ISS can only be
viewed either in dawn or dusk, depending on the moment of overflight. There are more factors playing
a role, and many things can be explained, but let's keep it simple for the moment.. Sofar the keywords are:
On this page I will try to explain satellite (or better: ISS) visibility.
I do this from my own views and experience.
What is needed to see ISS?
First of all, ISS must actually be above the horizon. This is the easy and logical part.
ISS is a huge structure, and the reason we can view it from the Earth's surface is because the SUN
shines it's light on it. However, against a daylight sky, our eyes will never catch it. So it needs to be dark.
Here is another problem: ISS is only illuminated by the sun, when it can 'see' the sun, so when the sun
is not behind the earth.....
In general this means that the sun cannot be too far below the horizon. Or better: ISS can only be
viewed either in dawn or dusk, depending on the moment of overflight. There are more factors playing
a role, and many things can be explained, but let's keep it simple for the moment.. Sofar the keywords are:
- sun must be set (but not too deep)
- it must be dark, but not too dark
- ISS must be above the horizon
- and no clouds or overcast conditions
In general one can say, that an
overflight takes 8 to 10 minutes, as
seen from the viewpoint.
ISS cycles at around 400 km altitude, and needs slightly over 1,5 hours to complete a cycle around the earth.
This means: if you're lucky, you can see it twice during a dusk or dawn period. That is of course.... when
weather is favourable. And that is not always the case in Holland. Unfortunately.
Why do ISS overflight times change?
ISS's circulation around the earth is chosen such, that not a complete number of revolutions is completed in
24 hours. In fact, approximately 15 1/4 revolutions are made during a day, each approximately one and a
half hour in time, at an altitude of around 375 kilometers.
Now, if ISS would have made an exact number of revolutions in 24 hours, it would end up above the same
spot every day at the same time. Since it doesn't, this means that each day the ISS overflight shifts slightly
'back' in time. That's why visibility of ISS happens earlier every day. Until a cycle is completed.
I made an extra page to go deeper into the programs related to sat watching (Orbitron, WXtrack):
Why is ISS visible nightlong in summer?
ISS's revolutions are alike. In fact, the only thing that changes is the Earth, circling below the Space Station.
Just think of it like this, and things become clearer. In summertime, at least in Holland, the sun will not sink
too deep below the horizon.
This means that in the May/June/July time frame, ISS will see the Sun, even in the middle of the night.
So, when it's dark in Holland, one will always see ISS during the night hours, no matter what time.
As long, of course, ISS will overfly Holland during night hours. See this:
ISS overflight at night in summer
Here is a nice example about the visibility of ISS during a summer night. This situation is for around 2 o'clock
in a July night in 2003. The yellow sinusoid line is the daylight corridor, Amsterdam is sleeping, ISS is
overhead, and the sun shines on ISS, because the visibility circle (the red one) is partly in the daylight area.
So: ISS sees the sun and we (if awake, and no clouds present) will see ISS.
Back to the 'satellite' page or Home
ISS cycles at around 400 km altitude, and needs slightly over 1,5 hours to complete a cycle around the earth.
This means: if you're lucky, you can see it twice during a dusk or dawn period. That is of course.... when
weather is favourable. And that is not always the case in Holland. Unfortunately.
Why do ISS overflight times change?
ISS's circulation around the earth is chosen such, that not a complete number of revolutions is completed in
24 hours. In fact, approximately 15 1/4 revolutions are made during a day, each approximately one and a
half hour in time, at an altitude of around 375 kilometers.
Now, if ISS would have made an exact number of revolutions in 24 hours, it would end up above the same
spot every day at the same time. Since it doesn't, this means that each day the ISS overflight shifts slightly
'back' in time. That's why visibility of ISS happens earlier every day. Until a cycle is completed.
I made an extra page to go deeper into the programs related to sat watching (Orbitron, WXtrack):
Why is ISS visible nightlong in summer?
ISS's revolutions are alike. In fact, the only thing that changes is the Earth, circling below the Space Station.
Just think of it like this, and things become clearer. In summertime, at least in Holland, the sun will not sink
too deep below the horizon.
This means that in the May/June/July time frame, ISS will see the Sun, even in the middle of the night.
So, when it's dark in Holland, one will always see ISS during the night hours, no matter what time.
As long, of course, ISS will overfly Holland during night hours. See this:
ISS overflight at night in summer
Here is a nice example about the visibility of ISS during a summer night. This situation is for around 2 o'clock
in a July night in 2003. The yellow sinusoid line is the daylight corridor, Amsterdam is sleeping, ISS is
overhead, and the sun shines on ISS, because the visibility circle (the red one) is partly in the daylight area.
So: ISS sees the sun and we (if awake, and no clouds present) will see ISS.
Back to the 'satellite' page or Home