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View Full Version : Just saw a dot in the sky move. What was it?



tgoolsby2
2012-Jan-04, 02:36 PM
I was up looking for the meteor shower earlier, and I saw what appear to be a star moving. When I saw it, it was already moving, but I watched it, and it eventually stopped moving. From my point of view, it was moving in a straight line. It definitely moved relative to all the other stars. From the time I looked at it to the time it stopped, I would guess it covered about 1 degree arc length of the sky. If I hold my arm straight out in front of me, the length of my index finger is equal to the distance it covered from my perspective. It covered this distance in about 30 seconds. However, it might have been moving before I had even looked at it. The "star" was quite dim. I could barely see it if I looked at it with the center of my vision (using my cone cells). I had to use my peripheral vision (my rod cells) to see it more brightly.

I would guess this is an artificial satellite, but I'm not sure because it looked like it stopped moving. The other possibility is I caught a glimpse of a rocket going up, and it looked like it stopped moving because the farther out the rocket gets, the less arc length it will travel. But can a rocket look like a star?

NEOWatcher
2012-Jan-04, 02:47 PM
I would guess this is an artificial satellite, but I'm not sure because it looked like it stopped moving.
We would need to know your location, exact time, and direction you were looking to give you some information.

If you want to check it out yourself, you can go to Heavens-Above.com to check for satellite passes. We can help you figure out how to use the site if it's not that apparent to you how to check it.

As for the stopping. It could be that there was a star there all along, and since you were concentrating on the satellite, you never noticed it. So; when the satellite reached the position of the star, and possibly faded out at the same time, then your mind replaced the two dots.

grapes
2012-Jan-04, 03:17 PM
The length of your index finger at arms length is probably more than five degrees, but still, that seems like odd behavior for a satellite. Are you sure it wasn't a distant plane that turned towards or away from you?

Jeff Root
2012-Jan-05, 08:14 AM
A few years ago I was looking at the stars, when the red,
blinking belly light of a high-flying jet (I presume) appeared
almost directly overhead. The second flash was, as nearly
as I could tell, right at the zenith-- straight above me. There
were no more flashes. No visible cabin lights, no reflection
from lights on the ground, and no jet noise. Absolutely quiet.
My sister, a few feet away, looking in roughly the same
direction, saw only the second flash.

No way can a satellite move a degree or more in 30 seconds
and then stop moving. NEOWatcher's guess that you lost a
moving satellite and replaced it with a star in that location is
plausible. Another possibility is the autokinetic effect, in which
a point of light on a dark field can appear to move:

http://en.wikipedia.org/wiki/Autokinetic_effect

For the case of a light bright enough to be seen by photopic
vision (color-seeing cone cells) surrounded by objects visible
only to scotopic vision (low-light rod cells), I discovered a
similar illusion myself. Evidently scotopic vision works faster
than photopic vision. When an object moves in your field of
view, the message from your rod cells reaches your concious
mind about a tenth of a second before the message from your
cone cells. When your eye moves slightly, which it does very
frequently in order to function, everything in your field of view
changes position by the same amount at the same time. But
you see the dim objects move immediately, and interpret
them as standing still. A lone bright object is not detected
as moving along with the dim objects, so you interpret it as
moving against the background of dim objects.

I first noticed this when playing with a calculator that had a
red LED display in almost complete darkness. There was
just enough early morning twilight to see the calculator case
and keyboard with my scotopic vision. When I suddenly
rotated the calculator in my hand, it looked as though the
LED display had come loose and was not rotating at the
same time as the rest of the calculator.

This might fit your situation with one star just bright enough
to see with your photopic vision, and the rest of the stars in
your immediate field of view visible only by scotopic vision.

-- Jeff, in Minneapolis

Jeff Root
2012-Jan-05, 08:38 AM
The width of a typical pencil is about 1/2 degree at arm's length--
the same as the diameter of the Sun and Moon. An adult male
finger would be more than twice that width-- more than a degree
wide. My index finger is about 3 inches long (hard to decide
where to measure to). From my eye to my finger is 26 inches.
So the angle is six and a half degrees.

-- Jeff, in Minneapolis

tgoolsby2
2012-Jan-05, 03:29 PM
NEOWatcher's suggestion is interesting. I'll check out Heavens-Above.com when I get the time.

I guessed the angle wrong, but the description i gave with my finger as a measure is accurate.

I've seen the affect Jeff Root described on a calculator, but I don't think what I saw was an optical illusion. The distance the dot traveled was too much, and it wasn't jittery.

No telling if it was a plane. There were no blinking lights. Best description of what it looked like is a dim star.

This was seen in Monterey, California between 5am to 6am local time toward the south-south-east. It was about halfway between the horizon, and looking straight up. The dot moved somewhat parallel to the horizon.

NEOWatcher
2012-Jan-05, 04:25 PM
NEOWatcher's suggestion is interesting. I'll check out Heavens-Above.com when I get the time.
From my own curiosity, I looked it up myself.
http://www.heavens-above.com/allsats.asp?lat=36.600&lng=-121.894&alt=23&loc=Monterey&TZ=PST&Date=40912.6041666667&Mag=4.5&AM=T
There seems to be several candidates to match your description.
What I don't see though is any of those sightings that fade out before reaching the 10 degree altitude mark.
(for example the COSMO-SKYMED 1 sighting faded out at 16 degrees, although that one went to high and in the wrong direction to fit your description)

But that doesn't rule out the possibility it was starting to dim at the point you saw it "stop". Nor does it rule out your eye being mis-directed to a star.

Jeff Root
2012-Jan-05, 04:47 PM
A high-flying bird is a possibility, too.

You've probably read this before, but when I was looking
at Comet Hyakutake through binoculars, a huge, silent,
triangular craft glided across my field of vision. It was
outlined by faintly-glowing, phosphorescent, oval-shaped
lights around the edges. I was totally flabbergasted.
A minute or so later a second craft, identical to the first,
followed on the same track. The second craft, however,
did something that the first did not: It honked.

-- Jeff, in Minneapolis

Cougar
2012-Jan-05, 08:25 PM
I was up looking for the meteor shower earlier, and I saw what appear to be a star moving.

Or it might have been the wonderfully named Phobos-Grunt satellite (http://legault.perso.sfr.fr/phobos-grunt.html), which is on its (unplanned) way back to Earth.

Chew
2012-Jan-06, 12:57 AM
I've seen the affect Jeff Root described on a calculator, but I don't think what I saw was an optical illusion. The distance the dot traveled was too much, and it wasn't jittery.

Were there clouds out when this happened? The autokinetic effect is easy to induce, with or without clouds, just stare at a star and sooner or later it will appear to start moving. The next time there are clouds out stare at a cloud and the nearby stars will appear to move.

tgoolsby2
2012-Jan-06, 03:35 PM
Or it might have been the wonderfully named Phobos-Grunt satellite (http://legault.perso.sfr.fr/phobos-grunt.html), which is on its (unplanned) way back to Earth.


From my own curiosity, I looked it up myself.
http://www.heavens-above.com/allsats.asp?lat=36.600&lng=-121.894&alt=23&loc=Monterey&TZ=PST&Date=40912.6041666667&Mag=4.5&AM=T
There seems to be several candidates to match your description.
What I don't see though is any of those sightings that fade out before reaching the 10 degree altitude mark.
(for example the COSMO-SKYMED 1 sighting faded out at 16 degrees, although that one went to high and in the wrong direction to fit your description)

But that doesn't rule out the possibility it was starting to dim at the point you saw it "stop". Nor does it rule out your eye being mis-directed to a star.

Here are other possible candidates I found off heavenly bodies besides Phobos-Grunt and COSMO-SKYMED 1: H-2A R/B, Cosmos 1378, Cosmos 1674, Resurs 1-4 Rocket, Cosmos 1238 Rocket. Some of these were moving moving more parallel than others to the horizon.

Also I forgot to mention, when I first was the dot, it was more towards the south-east, and then when it 'stopped' or faded, it was more towards the south-south-east.

Do you know where I can get a picture of what the night sky would have looked like in my area at that time?

Offtopic: Why is it that most of the satellites can't be seen below 10 degrees? Something to do with the atmosphere?


A high-flying bird is a possibility, too.

You've probably read this before, but when I was looking
at Comet Hyakutake through binoculars, a huge, silent,
triangular craft glided across my field of vision. It was
outlined by faintly-glowing, phosphorescent, oval-shaped
lights around the edges. I was totally flabbergasted.
A minute or so later a second craft, identical to the first,
followed on the same track. The second craft, however,
did something that the first did not: It honked.

-- Jeff, in Minneapolis

I think it's possible. The sun was just about to start lightening up the sky, but the bird might have been high enough to be able to see the sun.


Were there clouds out when this happened? The autokinetic effect is easy to induce, with or without clouds, just stare at a star and sooner or later it will appear to start moving. The next time there are clouds out stare at a cloud and the nearby stars will appear to move.

Nope no clouds, it was very clear.

With the autokinetic effect, how far could the dot have appeared to move? And how 'stable' would the dot appear to be? I've seen the autokinetic effect on a microwave's lcd screen in a dark kitchen, but the illuminated numbers seemed only slightly lag behind the movement of the non-illuminated stuff behind it. They never strayed too far from where they were supposed to be.

I'm guessing that the autokinetic effect with stars might be similar. The dot I saw moved in straight line at a uniform speed. It didn't seem jittery like the microwave lights did, and the distance it traveled seems like it would be too much for the autokinitic effect.

NEOWatcher
2012-Jan-06, 06:29 PM
Do you know where I can get a picture of what the night sky would have looked like in my area at that time?
On the link that I provided (which you quoted) for the passes at that time... You can click on the time that appears in the center of the pass for any of the satellite passes.
It will show you a sky chart for that date and time, along with superimposing the path for that particular pass. (little touches like that is why I love that site)

(West is shown on the right so you can hold it upside down against the sky)

Jeff Root
2012-Jan-06, 07:17 PM
Everyone should have a planetarium program, just like everyone
should hav a dictionary and an atlas. Maybe more than one
planetarium program: Just as you need different dictionaries
for different languages, and different atlases if you are interested
in highways versus watersheds, different planetarium programs
specialize in viewing the night sky from any location on Earth,
or viewing what the sky would look like from locations in places
far from Earth, or the paths of satellites overhead.

Unfortunately, the one program I am familiar with is very old,
and I don't know enough about the many, many newer ones
to make recommendations. BAUT *should* have a list of
them somewhere -- a very few are mentioned in:

http://www.bautforum.com/showthread.php/22865-**-FAQs-**-Resources-On-The-Web

including Celestia and Stellarium.

I've seen a web page listing at least 40 programs available for
several different computer platforms.

Such programs generally allow you to change your location,
direction of view, size of the field of view (magnification), the
number of objects visible (brightest only, or include very dim
stars, nebulae, and galaxies), and change the date and time
to any time within a few tens of thousands of years, at least.
They also optionally show constellation lines, the names of
objects, and miscellaneous info about them. Even my very
old DOS program, SkyGlobe, does all of that.

-- Jeff, in Minneapolis

Chew
2012-Jan-07, 12:48 AM
With the autokinetic effect, how far could the dot have appeared to move?

It would have appeared to move for as long as you looked at it. I've stared at stars for a minute and judging by the speed they appeared to move I would have sworn they should have moved 30. This is right after I going from a brightly lit room to the backyard and my eyes hadn't adjusted to the night sky so I couldn't see the fainter nearby stars.



And how 'stable' would the dot appear to be?

It would appear very stable; it wouldn't jump or jitter around.


I've seen the autokinetic effect on a microwave's lcd screen in a dark kitchen, but the illuminated numbers seemed only slightly lag behind the movement of the non-illuminated stuff behind it. They never strayed too far from where they were supposed to be.


Right. They appear to move but comparing it to other visible objects it is apparent they are not moving.


I'm guessing that the autokinetic effect with stars might be similar. The dot I saw moved in straight line at a uniform speed. It didn't seem jittery like the microwave lights did, and the distance it traveled seems like it would be too much for the autokinitic effect.

It can be deceptive. It is a very powerful illusion.

pbrekke
2012-Jan-07, 10:10 AM
You should see quite a few satellites moving across the sky if you just stare for a while.. 6000 satellites have been launched - 2000 is still left orbiting the Earth - and about 1200 is still operating...
So go Satellite hunting....and ISS hunting...