Y Can U C Satellites In The Sky?

[JimFox]

Why is it that you can see satellites flying over in night sky? What you are seeing is the sun reflecting off the satellite. But at night time the satellite is flying through the shadow of the earth, so no sun can reflect off of it, so you shouldn't be able to see it. Also why can you see them at all? They are hundreds of miles away (low orbit). I know they open up with solar cells and such, but still that's far away.

[beefymeg]

THANK YOU!! My friends insist you can see satellites, too, but again it doesn't make any sense to me why they would reflect the sun's light at night. I'm sure there's some other very logical explanation -

Oh, and you can't see them near any big cities - we're lucky enough to see any stars.

[pluto]

You can only see them if the sun is shining on them. You may be in the earth's shadow, but they aren't. There are portions of the satellite's orbit where it is in shadow, but the farther the satellite is from the earth (i.e., the higher the orbit) the less time it spends in shadow.

Part of the riddle as to why we can see something so small when it's so far away is that satellites have a very high [i]albedo[/]. Albedo indicates how well a body reflects light. Satellites are typically made of metal and/or foil-covered. The moon, in contrast (pun intended), has a very low albedo. It reflects light about as well as a dark, dusty rock.

In combination, the brightness of the sun, the shininess of the spacecraft and the darkness of the background end up making a visible point of light in the sky.


------------------
"non sunt multiplicanda entia praeter necessitatem"

[Nickrz]

Satellites are perfectly capable of reflecting sunlight "at night" as long as they are not in the earth's shadow. After all, our moon does it on a regular basis, n'est ce pas?

I remember seeing one of the very first man-made satellites (Telstar, I believe) in orbit when I was a child, and that sucker was not nearly as large as those thousands of more modern beauties they have up there today. Of course, the skies were very much darker back then, and NASA was so proud of their launch they disseminated detailed information on where and when to look.

The relatively rapid movement of a low-earth-orbit satellite against the backdrop of stars is fairly easy to detect with the naked eye.

However, I'm certain you'll never see a satellite in a geosychronous orbit unaided since it is too far away (22,000 miles) and very slow moving against the stars.

[JimFox]

Still I have seen satellites directly overhead at midnight. They must be in the earths shadow, (it seems the earths shadow would be pretty big, but I guess they can't be in the shadow if I saw them. I saw one the other day going from southeast to northwest and it was blinking on for a second then off for a second. It seemed to change colors to, there were 2 other people with me who saw it.

[Keeves]

Check out this site: http://shuttle-mir.nasa.gov/realdata/sightings/index.html NASA's computers predict exactly when and where to look for the US Space Shuttle (when it is in orbit) and the International Space Station. Used to do Mir too, maybe it still does.

Re: the question about the sattelites being in earth's shadow: This is indeed a problem, and you will see from the data there that all the sightings are between about 30-90 minutes before sunrise, or between 30-90 minutes after sunset.

For example, the Space Station will be visible on Thursday (8/26) night in London for about 2 full minutes. It will become visible 72 degrees above the horizon, in the western part of the sky, climb to a point 85 degrees above the horizon, and then descend, vanishing at a point 21 degrees above the eastern horizon.

I have seen it myself. You need a clear, cloudless night with no tall buildings in the way, but I have not found city glare to be a problem. Very cool!

[pluto]

A "blinking" satellite could be a slowly tumbling object with different reflectivity in different orientations. Typically this would be an upper stage booster that had enough energy to achieve orbit.

The highly inclined orbit makes it likely that this was a Russian launch, since they put a lot of satellites into high inclination orbits. This is partly because they launch from higher latitudes, but they also tend to put more satellites in low orbit than the U.S. In low orbit you have to move north and south to get the same coverage as you could from farther out. In geosynchronous orbit you are far enough out that an entire hemisphere is visible.


------------------
"non sunt multiplicanda entia praeter necessitatem"

[Undead Dude]

http://fathom.org/Stephen/sathunt.html

[bantmof]

But at night time the satellite is flying through the shadow of the earth, so no sun can reflect off of it...
Ah, but just because YOU are in the earth's shadow (hence, nighttime) doesn't mean the satellite has to be as well! "Night" is dependent on your location - even for people on the earth's surface, not everyone has night at the same time. By the same principle, it can be daytime for a satellite but nighttime for you who are looking at it. Since it is in daylight, but you're not, you can see the sunlight glinting off of it.

--
peas on earth

[Nickrz]

For example, the Space Station will be visible on Thursday (8/26) night in London for about 2 full minutes That is, of course, if those NASA guys can still get it up. I just returned from Florida, and was hoping to see my first launch ever, but NOOOooo.

Note to those who have trouble envisioning things still in sunlight while it's "night time" - ever see the top of a mountain range in full sun while you yourself are in the earth's shadow? Same thing applies to satellites, they being very much higher than a mountaintop and you being in deeper shadow than the previous example.

You can see the same effect by looking at earth's moon through a telescope or binoculars. Look along the edge of the terminator and you can frequently see apparently unsupported mountaintops jutting into the sunlight from the "dark side."

[Strainger]

[/quote]quote:
--------------------------------------------------------------------------------
For example, the Space Station will be visible on Thursday (8/26) night in London for about 2 full minutes
--------------------------------------------------------------------------------

That is, of course, if those NASA guys can still get it up. I just returned from Florida, and was hoping to see my first launch ever, but NOOOooo.[/quote]

Nickrz, it sounds like you're talking about the Space Shuttle. Parts of the Space Station are already up there.

[JimFox]

I guess I'm thick. Why can I see satellites at midnight (actually 11:00 Standard Time) directly overhead? Also does the Earth's shadow increase in size, get smaller or remain the same size?

[moriah]

Since the sun is much larger than the earth, there is projected 'behind' the earth a nearly pitch dark shadow that is conically shaped. It starts out the size of the earth's circumfrence and reduces to a point somewhere a little past the moon's orbit. This is the umbra (Latin for 'shadow'). When you're in the umbra, you can't see the sun at all. The moon is able to just fit into the small part of the umbra for a complete lunar eclipse (very rare).

Now, with the sun being large, and all that, there are portions of space where the earth blocks out part of the sun's light, but not all of it. This is the penumbra ('near-shadow' or 'almost-shadow' or 'next-to-shadow'). The penumbra is also conically shaped, but unlike the umbra, it starts at the earth's circumfrence and grows bigger, extending practically infinitely. When the moon moves into the penumbra, this is a partial lunar eclipse.

[The moon, also has an umbra and penumbra. Where the moon's umbra hits the earth, there is a total solar eclipse. Where the moon's penumbra hits the earth, there is a partial solar eclipse.]

Peace.

[Nickrz]

Oh yeah, I meant the shuttle of course.
I heard if you look closely at that station thing, (and you'll have to look closely because it might not get any bigger) you can see the Russkies laughing all the way to the bank.

[DSYoungEsq]

I guess I'm thick. Why can I see satellites at midnight (actually 11:00 Standard Time) directly overhead? Also does the Earth's shadow increase in size, get smaller or remain the same size?


Your not thick, Jim, you just aren't thinking three dimensionally. :)

'Directly Overhead' isn't necessarily the opposite of where the sun is. Remember, because the earth's axis of rotation is tilted 23+ degrees from 'vertical' the sun's position in the sky varies with the time of the year. At the summer solstice, the sun is as high in the daytime sky as it ever gets, and it is still not directly overhead anywhere in the continental US. If you think about it a moment, this clearly means it isn't directly below your feet, even at true mid-night in your location.

The conical complete shadow of the Earth (the umbra), while certainly big, isn't as big as you might think. First of all, even at the very edges of the Earth, it is only 8000 miles wide. We call that night, and use it to enjoy stars, junk in orbit, and that really noisy party down the street we didn't get an invite to. But if you were to extend even THAT circular cross section of the Earth straight out away from the sun, it would make a VERY tiny hole in the visible sky. Even at the distance of a low orbit satellite, it doesn't take many degrees off true 'up' to make the satellite visible. Hell, the MOON only manages to wander through that shadow every once in a while, and its orbit is roughly along the same plane as contains the sun and the earth.

If you want an interesting experiment in just how much shadow there is, see if you can locate a 'circum-polar' satellite as it makes its way north to south or south to north over you. If you catch it at the right time, you can watch the satellite enter the shadow of the earth, and dissapear, only to reappear some time later. I recommend getting some information in advance to help you do this, as it would be unlikely you would manage it unaided without considerable luck.

For a home experiment to make just what is going on clear, take a globe, and a flashlight into a dark room. Shine the flashlight at the globe from across the room, then start seeing in what positions an orbiting object is shaded from the light of the flashlight. You will quickly see even in this poor imitation that not all up is dark. :)

[pldennison]

There is a photo in the current Sky & Telescope in which someone managed to capture the paths of not one but six satellites during an extremely long exposure. Four are travelling approximately east-west, and two north-south.

[bantmof]

How do you know this was a satellite? Sounds like it could have been an airplane to me.
Indeed it can be almost impossible to determine what a point source of light at night actually is. You have no real basis for judging its distance - it could be very close and moving slowly, or very far away and moving rapidly - and it can also be almost impossible to judge how it is actually moving in 3-space, since you are just seeing a 2D projection of that movement, essentially. It can be so hard to tell what something is that even people who are experienced at it make a lot of mistakes.

--
peas on earth

[JimFox]

Thank you Moriah & DSYoungEsq for your detailed explanations. It seems to me, I would guess the Umbra and the Penumbra would be about the same size, the diameter of the earth, at the altitude of low orbit satellites, (200 - 400 miles above the earth?)(I would hope it has something to do with the ratio of the distance from sun to earth, in conjunction with the ratio of the distance from earth to object, then depending on that distance ratio, factor in the ratio of the sizes of the sun to earth.) So it seems to me that if it's dark here, it's dark up there too. Especially late at night. The ones I saw late at night I think went from west to east, horizon to horizon. I think they were at about 11:00PM Standard Time.
I will try the experiments that DSYoungEsq suggested, then maybe I'll see the light or in this case the shadow. Obviously I am wrong and I'm stuck in a 2 dimensional world.

[cmetzb]

Jim,
Another good site is http://www.bester.com/satpasses.html

I have been able to see MIR from very urban site.

[mblischke]

> Still I have seen satellites directly overhead at midnight. I saw one the other
> day going from southeast to northwest and it was blinking on for a second then off for
> a second. It seemed to change colors

How do you know this was a satellite? Sounds like it could have been an airplane to me.

Michael

[moriah]

DS,

A flashlight and globe doesn't make for a good model of the earth's shadow. When the light source is smaller than the object, the object's umbra is an expanding cone.

A flood light and golf ball would be better.

Peace.

[JimFox]

I doubt it could have been an airplane. The only time you can see an airplane in the sky, is when it's landing or launching lights are on. These are usually on during the last part of the decent or begining of the acsent. When I saw them late at night, I was 100's of miles from an major airport, in the Sierra's. Jets were so high, that no way could you see just their cabin lights.

[DSYoungEsq]

A flashlight and globe doesn't make for a good model of the earth's shadow. When the light source is smaller than the object, the object's umbra is an expanding cone.

True, but the point is that for most purposes, the narrowing of the earth's umbra is essentially irrelevant when discussing satellite shadowing. At the distance of even a geostationary satellite, the narrowing effect would be so small that it wouldn't matter much. My point with the experiment was to give him an easy way to see that the satellite can remain visible in the zenith because the shadow is so relatively small to the whole of the night sky.

I would do the whole math for you on the shadow thing, but as an attorney I hate working with numbers that don't have dollar signs in front of them ;)

[cmetzb]

Pl,
Which month of S&T does that picture appear.
I'm not doubting you, just interested.

[bantmof]

I doubt it could have been an airplane. The only time you can see an airplane in the sky, is when it's landing or launching lights are on. These are usually on during the last part of the decent or begining of the acsent.
Even airplanes landing at minor airports still use landing lights :-) I doubt you're 100 miles from _any_ airport.

I don't know what the object you saw was, but I would be hesitant to rule out airplanes altogether, since they're so common and at night they can look like they're doing things they're really not doing. And since human memory is so poor at recalling details, just because it seems like it was "straight up" doesn't mean it actually was.

As an aside, you can see a/c landing lights from much farther away than you might think. I've seen them from over 50 miles, and even 100 would not surprise me in the least.
--
peas on earth

[JimFox]

Peas,
Maybe your right, but I doubt it. There is a certain feeling to an airplane, as oppossed to a satellite. For one there are 2 lights.

[E1skeptic]

Nicrz...two strobe lights, one on the starboard wing and one on the belly under the wings.Not ALL commercial airplanes have the same lighting configuration, but modern aircraft usually have two RED strobes, one on "the roof" and one on "the belly", and two WHITE strobes, one on each wingtip. And you're right about them being useful to help pilots determine the orientation of incoming aircraft.

Now, JimFox, I have seen traffic (air traffic, that is) at MORE than 10 miles (60000 ft) from me. It's easy, especially if you have the right seeing conditions. Now, I'm not excluding the possibilty of you seeing a satellite at midnight, directly overhead, changing colors, and blinking. Just tell me a few things:
1. Where do you live (I need to know the latitude)
2. What were the "seeing" conditions? (air temperature, wind velocities, humidity, etc. at different altitudes)
3. Were you drinking? (alcohol, that is) KIDDING!!!

Seriously, it is possible to see satellites the way you have described it, it is just not very usual.

------------------
Men will cease to commit atrocities only when they cease to believe absurdities.
-Voltaire

[JimFox]

Ok,
It was Friday August 13th 1999 at 10PM Pacific Daylight Time, which makes it actually 9:00 PM. I believe more that it was a booster than a plane. I live 12 miles north of San Francisco, which is in California, maybe that explains it.

The satellites I saw overhead at midnight were years ago in the Sierra's on a dark night. They seemed to bve everywhere, Going West To East.

[bantmof]

this satellite was not in a landing pattern and was travelling way to fast
I'm also not discounting a satellite, but for a point source of light at night, it can be almost impossible to judge how fast it is going. All you know is its angular speed, but not how far away it is, so its linear speed could be almost anything. And even more than that, you're essentially just looking at a 2D projection of its movement, which can lead to all sorts of mistaken impressions about an object's speed and direction. Gut feel can _easily_ be totally wrong at night.

BTW, here is a good web page with a lot of info about observing satellites:

http://home.att.net/~janjeff/satintro.htm

It has quite a bit of info on when they are and are not visible, including the effect of the earth's shadow depending on your location and the satellite's orbital inclination.
--
peas on earth

[Nickrz]

Jim - Commercial aircraft are equipped with navigation lights that include a red and green on either wingtip, a red light on top, a white light on the tail and two strobe lights, one on the starboard wing and one on the belly under the wings.

These are designed, of course, to make the aircraft highly visible (primarily to other aircraft). Their placement on the plane also gives pilots a means whereby they can determine (hopefully) the physical orientation of the other plane and so take evasive or other corrective actions as necessary.

I live about 20 miles from O'Hare Field in Chicago, and I'm very familiar with the landing patterns. On a clear night, I can frequently see as many as five or six aircraft stacked in a pattern that stretches out 20 miles or more into the night sky, and the ones farthest away have not turned on their landing lights. Since commercial aircraft rarely fly above 40,000 or so feet, (7.4 miles) I think it's a safe bet you're seeing the lights of an airplane.
Especially if they are "blinking."
Especially if they "change color."

[JimFox]

I am also 20 miles from SFO. I see planes in their landing patterns all the time. For one it is obvious it's a plane, they are pretty low and their lights are bright, for 2 this satellite was not in a landing pattern and was travelling way to fast. The landing patterns is north to south, they take off to the south, this satellite was from south west to north west. I don't think you can see a plane up at 40,000 feet, strobes & all.

[Stephen]

quote
-----------------------------------------
So it seems to me that if it's dark here, it's dark up there too. Especially late at night. The ones I saw late at night I think went from west to east, horizon to horizon. I think they were at about 11:00PM Standard Time.
----------------------------------------
No doubt you've heard of the 'White Nights.' During summer, north of 60 degrees latitude, the Sun dips just below the horizon, but the sky never quite gets dark.

Orbiting satellites are easily above the Earth's shadow until the Sun has set more than 25 degrees below your local horizon. If you live North of say, 40 degrees latitude, even the lower East-West orbits are above the Earth's shadow all night long, since the sun never goes more than ~20 degrees below your local horizon.

I live in Florida (28 deg latitude) and during late June to early July, the ending 'Window' for viewing sats (in easily viewed 'naked eye' orbits) runs until 11:30 PM.

I'd like to thank UndeadDude for posting the link earlier. If anyone is interested in downloading a shareware program for predicting the passage of visible satellite passes (and plotting their path among the stars, among other features). I've written just such a program "Satellite Hunting". You can find it at http://stephen.fathom.org/sathunt.html (as mentioned in my sig). You can also find descriptions and graphics explaining many of the principles involved in visible satellite observing, as well as links to related sites such as the Visual Satellite Observer's Homepage and the SeeSatL email list. Happy Hunting!

[JimFox]

Am downloading sathunt111, boy this has become rather technical, I hope I can comprehend. Why can't things be simple? Or is that another topic?

[Stephen]

Am downloading sathunt111, boy this has become rather technical, I hope I can comprehend. Why can't things be simple? Or is that another topic?

I tried to keep it as simple as possible. IMHO, it is easier to get started with than any similar software that came before (most of which is DOS based and requires careful editing of (often cryptic) control files). If you have any questions, try the online help, or feel free to email me.

------------------
Stephen
stephmon@aol.com
http://stephen.fathom.org
Satellite Hunting 1.1.0 visible satellite pass prediction shareware available for download at
http://stephen.fathom.org/sathunt.html

[rowrrbazzle]

For just about everything you might want to know about major stuff still in earth orbit (including Vanguard 2 and some used rocket boosters), try

http://liftoff.msfc.nasa.gov/RealTime/JTrack/Spacecraft.html

Ya got yer sighting calculations, ya got yer ground tracks, ya got yer satellite information, ya got yer real-time 3D display of the satellites' positions in space.


Nickrz: was it actually Telstar you saw, or was it Echo? I remember seeing Echo many times in the early 60's.

When I first came across the above site, I checked out possible views for the brighter objects and found they were only visible two or three hours before dawn. Remembering Echo nearly overhead before midnight, I looked for info on it. I finally found the info on a Japanese space program page. As I recall from there, the Echo satellites (there were two) had orbits between 1000 and 2000 km high. The current bright objects are under 500 km, so they are more likely to be in the earth's shadow than Echo was.