Riding the sky

By Phil Plait | October 13, 2010 7:57 am

icstars_leonids_uluruTaking the trash out the other night, I did what I always do: I looked up. The crescent Moon was setting, and I could see the Summer Triangle easily (after a few minutes, when my eyes adjusted a bit, I could see the Milky Way piercing through, too).

As I walked back in, I happened to turn to the east. I could see the big square that marks the constellation of Pegasus, and to its lower left, the two curved lines of stars denoting Andromeda. Instantly, I saw something wrong. Andromeda doesn’t have two stars so close together, I thought. Ah, I realized a second later, one of them must be a satellite.

Sure enough, as I watched, one of the stars could be seen to be moving. I had a moment of disorientation when it was hard to tell which star was moving away from the other, but after a few seconds it was obvious which was the star and which was the satellite.

It looked to be about second magnitude to my eye — bright, but nothing special — and moving quite slowly, slower than the space station does at that altitude off the horizon. Must be in a higher orbit, I thought, maybe twice as high? A bit higher? It was moving to the southeast, I noted.

cosmos2219_passI went inside, washed my hands (I was taking out the trash, after all) and got online. Firing up Heavens-Above.com, I quickly found a passel of satellites that had just gone overhead. Looking at the sky tracks of each, one jumped out: Cosmos 2219, an old spent Russian booster rocket. The path across the sky matched what I saw pretty well.

Not only that, the orbit is listed as being about 830 km (500 miles) above the Earth’s surface. That’s much higher than the space station’s orbit of 360 km (215 miles). Sure enough, that would account for the slower moving satellite I saw; the higher up the orbit is, the weaker Earth’s gravity pulls on an orbiting object, and the slower it moves. And my initial guess of the actual orbital height wasn’t that far off.

And the pièce de résistance? As I watched the satellite move, I thought to myself, the Sun set over an hour ago. That means that right around that part of the sky — mentally flagging a region near where Jupiter was — the satellite’ll fade out.

Boom. Right on the money. Satellites are up so high that even when it’s dark on the ground, they are in sunlight. As they orbit, they can move into the Earth’s shadow and fade out. The height off the horizon of the edge of the shadow depends on how far below the horizon the Sun is (draw yourself a picture, or use a flashlight for the Sun and a ball for the Earth and experiment). So I was able to eyeball where the object would fade away, and like clockwork, it was right on schedule.

I know I’m a scientist, and something of a dork, but it’s still undeniable: science is everywhere. It’s all around us, if you just take a moment to notice. From things too small to see (washing my hands to get rid of any germs from the trash) from things almost too big to grasp (the Milky Way laid out above me, sprawling across the sky a hundred thousand light years wide).

Had I put the trash bin on the street and turned clockwise instead of counterclockwise, I might’ve missed that satellite, never predicted in my head when the Sun would set for it. But seen or unseen, it would have done what it did nonetheless. The Universe rolls on with or without us.

But as I stood there, as I had so many times before, it reaffirmed what I know, what I’ve known for so long:

This is a great ride.

I’m keeping my eyes open. I don’t want to miss anything.


Leonids over Uluru image courtesy Vic and Jen Winter at ICSTARS.

Comments (63)

  1. Jeff

    “I know I’m a scientist, and something of a dork, but it’s still undeniable: science is everywhere. It’s all around us, if you just take a moment to notice. From things too small to see (washing my hands to get rid of any germs from the trash) from things almost too big to grasp (the Milky Way laid out above me, sprawling across the sky a hundred thousand light years wide). ”

    excellent “Sherlock Holmes” deduction on that satellite.

    Yes, science is everywhere, I’ve told the college kids that for 30 years, and everyday I see science in action. All you have to do is open your eyes, and wallah, it’s there!

  2. Michael Aye

    You show such enthusiasm in your writing! I am thankful for your writings, science is just cool.
    Best regards,
    Michael

  3. Jason

    We were out the other evening looking at the stars and saw a satellite go almost directly overhead. I didn’t try to find which one it was.

  4. Kevin
  5. Thank you, Dr. Plait. I’m a math teacher in a midwestern high school, and we have a literacy plan to help our students become better readers, where EVERY teacher reads for >= 5 minutes to each class, with text support. Your blog (and specifically this post) is a gold mine of source material for me to read to my students. This post will be Friday.

  6. Messier Tidy Upper

    @1. Jeff & 2. Michael Aye : Seconded by me. :-)

    Nicely spotted and deduced and written, BA. :-)

    big square that marks the constellation of Pegasus,

    The Great Square of Pegasus which for those who don’t know already consists of :

    1. Markab (Alpha Pegasi) a B9 blue star 140 ly away and 205 x our Sun’s luminosity.

    2. Algenib (Gamma Pegasi) a B2 blue subgiant, 333 ly off and 4,000 x our Sun’s brightness – very slightly variable.

    3. Alpheratz or Sirrah (Alpha Andromedae – but formerly Delta Pegasi also!), a “shared star” between constellations eventually made into Andromeda’s lucida (brightest star) of spectral class B8, another blue subgiant which – if I recall right – has had some starspots on its surface measured & discussed on a past BA blog thread here? Sirrah / Alpheratz is 97 ly away, 200 x the solar luminosity and a spectroscopic binary with a companion star orbiting in 97 days.

    & finally

    4. Scheat (Beta Pegasi) an M2 bright red giant so large it’s surface would extend out 70% the way to Venus. Located 200 ly distant it is also a noticeable irregular variable with a half-magnitude range (mid 2nd to bright third mag. apparent)and has steam in a shell of gas surrounding it.
    (Just be very careful how you pronounce its name! ;-) )

    Source : Kaler’s Stars website – links to follow separately.

    The Great Square of Pegasus asterism is also a great landmark (skymark?) for finding other stars notably the front half (Markab and Scheat) point directly~ish but over a fair way to Fomalhaut and the back half (Algenib and Alpheratz) will via a curving diagonal line get you to Diphda or Baiten Kaitos (Beta Ceti) the tail of the celestial whale.

    Moreover, two faint but significant stars lie nearly exactly opposite each other roughly midway on a line from Markab down to Scheat in the front half of the Pegasan square. On the ouside of the square is 51 Pegasi which was the first main-sequence “normal” sun-like star with an exoplanet – the first Hot Jupiter ever found – confirmed to orbit it in 1995.

    Just across the imaginary Markab-Scheat line is HR 8799 which ha sseveral distinctions being the only star that is simultaneously a Gamma Doradus variable a Lambda Bootis metal poor star and has a Vega style protoplanetary disk around it. (This trio of distinctions has led me to dub it “Gadolabove”) However, HR 8799 far better known for being – jointly with Fomalhaut – the first star to have an exoplanetary system directly imaged orbiting it back in (from memory) 2008. “Gadolabove ” has three young superjovians circling it at vast distances, still glowing warmly and all captured on human camera! :-)

    Also the number of fainter stars you can see inside the Great Square is a useful indicator of the level of light pollution.

    Hope folks find that interesting / helpful. :-)

  7. Kevin

    I’ve been looking at the night sky since I was a little kid, and decades later I still love going out and observing; sometimes with a telescope and other times with the naked eye.

    I don’t know why everyone doesn’t love it.

  8. Messier Tidy Upper

    See :

    http://stars.astro.illinois.edu/sow/51peg.html

    For Kaler’s photographic labelled map showing the Pegasan square stars and 51 Pegasi and “Gadolabove” HR 8799′s locations.

    See :

    http://stars.astro.illinois.edu/sow/algenib.html

    for more on Algenib and links to its stellar stablemates, the other three Pegasan square stars.

    Incidentally, it turns out Alpheratz / Sirrah is the brightest “Mercury-Manganese” star in our sky (apparent mag~wise natch) and there’s a link here :

    http://www.newscientist.com/article/dn12134-weather-observed-on-a-star-for-the-first-time.html

    to stellar “weather” being observed on it via New Scientists which I think the BA blogged on too but cannot find yet.

  9. bigjohn756

    It must be nice to be able to see 2nd magnitude stars. I can’t do it from my house most nights.

  10. Timmy

    Taking the trash out the other night, I did what I always do: I looked up. I could see the moon and about three stars through all the light pollution. Then I realized two of those stars were helicopters. So I went inside and made a sammich.

  11. Just Monday night I was watching a Breeze-M tank swing by in St Petersburg, FL, and it was clearly moving at a much quicker pace than the standard orbital speed of ISS and Hubble.

    Small wonder – the tank has a perigee of 325 km and an apogee of 14,112 km. What’s the math for working out the speed of these highly elliptical orbits during perigee?

  12. Messier Tidy Upper

    HR 8799 which has several distinctions being the only star that is simultaneously a Gamma Doradus variable a Lambda Bootis metal poor star and has a Vega style protoplanetary disk around it. (This trio of distinctions has led me to dub it “Gadolabove”)

    My “Gadolabove” name coming from :

    Gamma Doradus (var.), Lambda Bootis (metal poor), Vega (dusty disk)

    Naturally. Just in case that wasn’t already clear.

    Still no luck despite finding a BA blog ref. to anything on Alpha Andromedae’s surface despite google. Sigh. My net-fu must be weak tonight. :-(

    It might’ve been another different reference to weather on exoplanets around Upsilon Andromedae instead that I’m confusing with that but I could’ve sworn not.

    @11.Timmy & 10. bigjohn756 : My sympathies, light pollution sure sucks. Hope you get a chance to view clear dark skies from somewhere often. :-(

  13. Warning.
    Off-topic.
    Apologies all round:
    :(

    Mr Plait, please call James Randi now.
    It’s happened again.
    The whole global warming denialism zombie thing has reared it’s ugly head.

    The latest article on the JREF website is a plug for the on-line Reason Magazine.

    This is bad.

    Reason Magazine is a platform of libertarian climate denialism.
    I don’t care about their political views but I do care about the climate denialism part.

    Our friends at Reason magazine have a cruise on the horizon! We would like to invite interested skeptics to join Reason’s all-star cast of investigative journalists, policy wonks, and assorted warriors for freethinking and free markets for a week at sea. It’s Reason’s first-ever seven-day cruise!

    When you click the link at the bottom and go to the Reason magazine website itself, you get this precious piece of wisdom as they advertize the very same cruise….

    Of course, there’s more to this cruise than just lazing on decks. You’ll hobnob with some of the most incisive writers and thinkers in America (…) Science Correspondent Ronald Bailey who can explain why the Florida coast we are sailing from will still be there a century from now no matter what global warming activists tell you.

    Yet there’s more…
    Type in “global warming” on their web-site and you get a host of articles poo-pooing global warming in general. Pretty much every single prominent climate denier out there gets a favourable mention at one time or another courtesy of Reason.com.

    These people are science deniers. Their political ideology is skewing whatever critical thinking skills they may or may not have.

    They are no friends of the JREF.

    Please, please, please say something. Bring this to somebody’s attention.
    Please.

  14. Don't Panic

    I’ve impressed my friends before showing them on my Starry Night software which satellite just flew over, its pretty fun, and makes you feel super nerdy.

    Not all that long ago my Mother was telling me about a UFO that she saw from her hot tub last winter. “It was moving along and then just went out! Just like that it disappeared!” She was frustrated with my skepticism, and not entirely convinced with my explanation that it was a satellite that simply was high enough to be in the sunlight and then passed into shadow. Just another example of how people can jump to conclusions regarding UFO’s.

  15. AtomicTommy

    After reading all that, I could hear Jack Horkheimer in my head telling me, “Keep looking up!”. I miss that guy…

  16. AliCali

    Question:

    “…the higher up the orbit is, the weaker Earth’s gravity pulls on an orbiting object, and the slower it moves.”

    Is it really weaker gravity the biggest reason that causes the satellite to appear to move slower, or just that it’s so high up? Although gravity is weaker, I thought the distance is the main reason the satellite seems to move slower to us down here. For instance, an airplane traveling at 500 mph seems to move a lot slower when it’s 30,000 feet up than if it were right next to us somehow.

    (And that’s right, I used mph and feet. I don’t use the “metric” system; I use the American (or “correct”) system.)

  17. Messier Tidy Upper

    Plus also off topic but of interest – in case folks haven’t seen on the relevant sidebar yet – this sounds like bad news :

    http://blogs.discovermagazine.com/80beats/2010/10/12/um-that-goldilocks-exoplanet-may-not-exist/

    For Gliese 581 g

    which may may not actually be

    after all .. :-(

    Durnnit!

    Still on the bright side at least we hadn’t yet launched a spaceprobe to it .. with people aboard. ;-)

    Plus the Gliese 581 system still remains one of the most fascinating and promising we’ve found so far even without that “goldilocks planet.”

  18. swillyroot

    This was a beautiful little piece, Mr. Plait. Thank you.

  19. Tim G

    While camping in Vermont in the summer of 1994 I saw a “star” move slowly across the sky. It was moving far too slowly and persistently to be a meteor. It’s brightness was steady so I thought an aircraft was unlikely. I was still reluctant to conclude it was a satellite because it seemed like it was during the middle of the night. It was rather eerie watching it. But, I suppose in Vermont during the summer the sun does not dip too far below the horizon even during late hours.

    I agree with AliCali. The speed of an object orbiting 500 miles up is not too much slower than an object orbiting 200 miles up.

  20. I second AliCali’s nerdnit.

    Gravity has no affect (well…) on the speed of the rocket booster, which was set by the power of the engine that shot it up there in the first place. The only thing slowing the rocket down would be atmospheric drag, which at that altitude would be next to nil.

    The booster appears to be going slower because it’s farther away. It has a greater distance to travel to cover the same apparent angle as a satellite in a lower orbit.

    Besides, it was an alien spacecraft anyway.

  21. Pedantic Asshat From Space!

    Very nice article.

    There should be a moratorium placed on the term, “I don’t want to miss a thing,” or any slight variations of it in astronomy circles though. At least until Armageddon fades from humanity’s collective memory.

  22. QuietDesperation

    I know I’m a scientist, and something of a dork,

    Here I go again.

    Never call yourself names and NEVER take an apologetic stance for being interested in intellectual things! It just perpetuates the stereotype placed on us. That’s why I refuse to embrace the terms geek and nerd. Don’t you think that plays into the anti-science mood that’s infesting our society these days? It adds to the marginalization.

  23. J

    Damn, where the hell do you live? Must be nice.

  24. AliCali and kuhnigget: Satellite orbital speed is determined by the gravity of the object it is orbiting. Yes, a rocket had to put it there, but the speed the rocket gives it so that it orbits is determined by gravity. Give a satellite too much speed and it escapes; don’t give it enough and it comes back. Give it the right amount, and it will orbit.

  25. Gary

    “I’m keeping my eyes open. I don’t want to miss anything.”

    Not missing anything takes more than eyes. It takes challenging deeply held convictions every once in a while. Self-blinding is more common than we think…

  26. Edward Carney

    More awe. Have a look at “Uluru, Northern Territory, Australia” as it appears on Google maps satellite view. Makes you want to visit.

  27. Zucchi

    Seconded about Reason Magazine. I hate the way they twist the word “skeptic” to mean just about the opposite of what it’s supposed to mean.

    Man, I’ve got to get out of Atlanta and go someplace where I can see the stars (and satellites) at night. Haven’t seen the Milky Way in years.

  28. Oh boy! I get to spar with the good doctor!

    …the higher up the orbit is, the weaker Earth’s gravity pulls on an orbiting object, and the slower it moves.

    Satellite orbital speed is determined by the gravity of the object it is orbiting.

    Mmmm….sorry, but I think the original wording is still incorrect. The weaker gravity of a higher orbit does not impart any lesser amount of speed to the satellite. I know, I know, it seems pedantic, but it is the rocket booster that imparts the speed, not the gravitational field of the earth. The required velocity to place and keep a satellite in orbit is determined by the gravity, but again, that gravity does not actually impart any speed to the satellite. (It “accelerates” the object, because it is constantly changing direction and a change in direction is a form of acceleration, but it doesn’t affect the speed.)

    I think what you mean is, the satellite was boosted into a higher altitude, at which the earth’s gravitational pull is weak enough so the satellite’s slower speed was still sufficient to keep it in orbit. But again, the earth’s gravity didn’t put it there. It just keeps it there.

    So…

    …the higher up the orbit is, the weaker Earth’s gravity pulls on an orbiting object, and the slower it needs to move.

    Now pardon me while I go clean up.

  29. Calli Arcale

    Gravity has no affect (well…) on the speed of the rocket booster, which was set by the power of the engine that shot it up there in the first place. The only thing slowing the rocket down would be atmospheric drag, which at that altitude would be next to nil.

    Gravity certainly does have an affect on its speed; as Phil said, the orbital velocity really is different at different altitudes — if you aren’t at the right speed to orbit at a certain altitude, you simply won’t. It’s all ballistics. Think of it this way: being in orbit means traveling forward just fast enough that as gravity pulls you down, you miss the Earth and fall right around it. If you are traveling slower, your curving arc will intersect the Earth, right? Well, the opposite condition works too — if you’re traveling faster, your curving arc will widen, and you won’t be in that neat, circular orbit anymore. But here’s where it gets weird. Objects travel faster near perigee than at apogee, and even an escaping object is decelerated on its way out, because of the Earth’s gravity pulling it back.

    Navigating in space is weird. It’s not at all like navigating on Earth. You don’t drive to an orbit and then park. In spaceflight, the adage is “speed up to slow down, slow down to speed up.” It’s all very counterintuitive, but it works. And it’s really just ballistics. Throw a few rocks to get a feel for it; the rock slows down as it rises, and this has very little to do with aerodynamics (unless you’re throwing really hard or the rock is a really bad aerodynamic shape).

    Phil isn’t kidding when he says science is everywhere. It’s even in a thrown rock. ;-)

  30. *waves*

    Small issue with: ” Sure enough, that would account for the slower moving satellite I saw; the higher up the orbit is, the weaker Earth’s gravity pulls on an orbiting object, and the slower it move.”

    “Earth’s gravity” = g?

    Or did you mean the gravitational force between them? As in F = GMm/r**2

    (I’m an astrophysics student)

    I’ve been staring at it trying to figure out how g fits in.

    v = sq rt of (GM/r)

    I think that may be what is causing the confusion above.

    Being a bit of a math dork =P standard convention is awfully important to me.

    g /=/ G tis a bit basic but may need to be stated.

  31. @ Calli Arcale:

    Heh heh… My “out” is in the “(well…)” cleverly placed within my first sentence. A ruse to avoid the details you bring up!

    BUT…my point still stands. That gravity did not impart speed to the satellite. It set the bar regarding how hard the rocket booster had to work, but it was the rocket that did that work.

    But, oh, I’ve run out of nits!

  32. Wayne on the plains

    Kuhnigget:
    While Phil’s original wording and subsequent explanation may not sound right to you, he is correct in that only one speed will keep a satellite in a particular circular orbit, and that speed is determined by the strength of gravity.

    One aspect he didn’t directly address is the effect of distance on how fast it APPEARS to be moving (as opposed to how fast it is actually moving). Without getting into the actual math, you can show that the actual, linear speed required for circular orbit falls as one over the square root of the radial distance (from the center of the Earth). Angular speed (how fast it moves across the sky) is velocity divided by radius, so neither effect is negligible in this case.

  33. Calli Arcale

    The required velocity to place and keep a satellite in orbit is determined by the gravity, but again, that gravity does not actually impart any speed to the satellite. (It “accelerates” the object, because it is constantly changing direction and a change in direction is a form of acceleration, but it doesn’t affect the speed.)

    Nope, gravity really does change the speed. It slows the receding satellite. It really does. It doesn’t just change the direction. This is hard to grok; it’s why heliocentrism had such a hard time getting accepted by the mainstream, because it took a long time for anybody to figure this part out.

    Speed up, and you’ll increase your orbit, and slow down.
    Slow down, and you’ll decrease your orbit, and speed up.
    Sit in an elliptical orbit, and you’ll get to speed up and slow down repeatedly over the course of each orbit.
    Weird but true.

  34. Jason

    Wait.. You mean orbiting doesn’t occur when intelligent falling isn’t quite smart enough?

    *grin*

  35. Calli Arcale

    Ah, you posted while I typed. ;-)

    BUT…my point still stands. That gravity did not impart speed to the satellite. It set the bar regarding how hard the rocket booster had to work, but it was the rocket that did that work.

    The rocket did the work, yes, but the velocity is actually determined by a combination of the rocket’s work and the Earth’s gravity.

    New Horizons is an interesting example. It had the fastest velocity at launch of any spacecraft, ever — at the final engine cut-off, it was going 16.26 km/s (relative to the Earth, per Wikipedia). It is presently going 15.97 km/s (relative to the Sun). Now, I’ve cheated a little — those are relative to different bodies. But they’re the only figures I could get. Point is, New Horizons has decelerated. It is still decelerating, even on its escape trajectory. The Sun is doing more than just bending its trajectory. It’s actually slowing it down. It just won’t slow it down enough to recapture it.

  36. @ Wayne on the plains:

    While Phil’s original wording and subsequent explanation may not sound right to you, he is correct in that only one speed will keep a satellite in a particular circular orbit, and that speed is determined by the strength of gravity.

    Yeah, I get that. Really, I do! :)

    @ Callie Arcale:

    Speed up, and you’ll increase your orbit, and slow down.
    Slow down, and you’ll decrease your orbit, and speed up.
    Sit in an elliptical orbit, and you’ll get to speed up and slow down repeatedly over the course of each orbit.
    Weird but true.

    Yeah, yeah, I get that, too! (It’s in that “(well…)” again!)

    BUT…the acceleration and deceleration (caused by gravity!) of a satellite in an elliptical orbit is not what what the good doctor was describing in his original sentence:


    the higher up the orbit is, the weaker Earth’s gravity pulls on an orbiting object, and the slower it moves.

    See? The higher orbit and the weak gravity therein does not cause the satellite to move more slowly. The satellite is in that higher orbit because a rocket engine burned long enough and powerfully enough to impart the speed required to get it there.

    Now I really do need to go wash off. :P

  37. AliCali

    Okay, I get that the gravity is weaker at higher orbits. And I get that the required speed to maintain orbit is thus lower.

    However, I maintain that major reason the satellite is APPARENTLY moving slower FROM OUR PERSPECTIVE is not due to slower speed, but rather due to the fact that the satellite is further away. As Kuhnigget said, the satellite “has a greater distance to travel to cover the same apparent angle as a satellite in a lower orbit.”

    To extend my analogy, the airplane traveling at 100 mph on the ground next to me seems to be going much faster that one traveling at 500 mph way up in the air.

    I suppose I could calculate the effect of slower speed vs. distance to see which contributes more to the slower movement, but I’m buried in tax returns and too far removed from my physics classes.

  38. Calli Arcale

    Yes, it is! And I get what you’re saying now. I’m inclined to say Phil’s wording was, at best, misleading, but not actually wrong.

    AliCali — it would be cool to see that math. ;-) Good luck with your tax returns! Far less personally satisfying, but much more important to get right.

  39. Jeff

    kuhnigget:
    “Gravity has no affect (well…) on the speed of the rocket booster, which was set by the power of the engine that shot it up there in the first place. The only thing slowing the rocket down would be atmospheric drag, which at that altitude would be next to nil.”

    yes, and that is why the Vanguard I satellite, launched by US Navy on March 17, 1958, just 6 weeks after Explorer I, the first US satellite, has a (a) apogee of 6500 miles (b) perigee of 4400 miles and is not expected to fall back to earth until after 2150, although that date is uncertain.

    “oldest satellite” : http://en.wikipedia.org/wiki/Vanguard_1

  40. Dave Regan

    Speed of orbits with numbers.

    In low Earth orbit, an orbit takes about 90 minutes. The earth is about 25,000 miles in circumference giving a velocity of about 17,000MPH and has a ground speed of about 17,000MPH. It will appear to cover 1/4 of the orbit in about 22 minutes.

    GPS satellites are at about 16,600 miles from the center of the Earth, and the orbit takes about 12 hours for a circumference of about 104,000 miles, and a velocity of about 8700MPH. It will cover 1/4 of the orbit in about 3 hours.

    In Geosynchronous orbit, the orbit takes 24 hours. The orbit is about 26,199 from the center of the Earth, or about 164,600 miles in circumference or about 6858MPH with a ground speed of about 1000MPH matching the Earth’s rotation. This won’t appear to move from the Earth.

    The moon is about 242,857 miles from the center of the Earth. The circumference is about 1,526,000 miles for a velocity of about 2300MPH with a ground speed of 37MPH. This covers (backwards) 1/4 of the orbit in about 7 days.

    You can interpolate for other cases of circular orbits.

    You can get more accurate estimates for any of the numbers that I have used.

    The fact that the Earth rotates makes some of the numbers more interesting.

    The 1/4 of the orbit is just a simple metric to give angular speed while you’re lying on the ground and looking at the sky.

    I probably messed up the math at least once in there.

    Dave Regan

  41. Jason

    Let me see If I understand the reference to ground speed correctly.

    If I were to shine a laser straight down onto the earth’s surface and keep it pointing so that it formed a 90degree angle with the local horizon it would appear to move across the ground at the above speeds? Or am I seeing this wrong?

  42. Dave Regan

    Jason #43:

    What I was trying to get with ground speed was that if you were in a car moving at the stated speeds, you could keep the satellite directly overhead.

    My numbers are somewhat ambiguous because of the rotation of the Earth. Obviously, geosynchronous has a ground speed of 0 if you are an observer on Earth, but the satellite does orbit the 25,000 miles of Earth during the 24 hours so it sort of has a ground speed of 1000 if you negate the Earth’s rotation.

    For the 90 minute orbits, the Earth’s rotation is insignificant. For the lunar orbit, the Earth’s rotation is insignificant. For the GPS orbit and geosynchronous orbit, the Earth’s rotation matters.

    Dave Regan

  43. Brian E

    I do the same thing every time I take out the trash. I can’t lie, you’ve definitely been a factor in that, spiking my interest in the heavens dramatically. Thank you, Phil.

  44. Jason

    @Dave regan,
    that is basically the same thing .. Thats what I thought.

  45. Wayne on the plains

    As long as we’re putting numbers to things, let me grab that envelope that I did my original math on (no, really I did).

    If you could orbit at the Earth’s surface (don’t try this at home) the velocity is 7.9 km/s, so as stated, the 0.3 km/s rotation of the earth is minor. The angular velocity is only 0.0012 rad/s, or about 4.3 deg/min. Low Earth orbit is similar, but slightly less.

    At 500 km up, the velocity is 7.6 km/s (3.7 % less), but the angular velocity is 3.8 deg/min (12 % less).

    In general the decrease in angular velocity (how fast it moves across the sky) is much bigger than the decrease in linear velocity (because v goes down as one over square root R and angular velocity goes down as one over R to the three-halves power).

    So… Yes, being farther away is more important than the satellite having a slower orbital speed as far as making it look slow.

  46. ET

    science is everywhere

    but so are economics, politics etc

  47. Gary Ansorge

    14. Cedric Katesby

    As far as Reason Mag adverts are concerned, every time you click on it, money goes to the JREF.

    So, I think I’ll wander over to JREF and click on it about a million times.

    ADDENDUM, ALERT, WARNING WILL ROBINSON:

    “Science Correspondent Ronald Bailey who can explain why the Florida coast we are sailing from will still be there a century from now no matter what global warming activists tell you.”

    Funny, when I went to their cruise page, I saw no reference to global warming.

    Are you trolling?
    Additional thought: since we don’t know how bad things will get, even if all of the Greenland ice melts, it will only raise sea level 6 meters. I don’t think that’s enough to inundate all of Florida.
    Gary 7

  48. @ Gary Ansorge:

    Additional thought: since we don’t know how bad things will get, even if all of the Greenland ice melts, it will only raise sea level 6 meters. I don’t think that’s enough to inundate all of Florida.

    True. I suspect the top of the Citrus Tower would remain above water.

    Also, all those sinkholes would drain away a lot of the rising ocean.
    ;)

  49. “Science Correspondent Ronald Bailey who can explain why the Florida coast we are sailing from will still be there a century from now no matter what global warming activists tell you.”

    Funny, when I went to their cruise page, I saw no reference to global warming.

    Go to the JREF article.
    Here.
    Click the link to reason.com at the bottom.
    It take you to this page.

    The article from where the quote is taken is titled..” I love it when we are cruising together” and is written by Matt Welch, Editor-In-Chief.

  50. Brian Schlosser

    The Universe might roll on without us, but if We weren’t here, there would be no Cosmos boosters prettying up the night sky! :D

  51. Kaso

    I remember the first time i saw an iridium flare, i planned to see it so me and my partner were laying out on the grass, but seeing the flare appear on time to the second, wizz across the sky then vanish on time to the second, really made me appricate the maths involved in these things

  52. Tim G

    A satellite in a circular orbit 215 miles up should be moving around 7704 m/s. Not factoring in Earth’s rotation, the apparent motion would max out at 1.276 degrees per second. A satellite 500 miles up would move at 7454 m/s, translating to a maximum apparent speed of 0.531 degrees per second. The slower apparent speed is due more to the altitude than the actual speed, which is the point AliCali was making.

  53. Tribeca Mike

    Satellites truly are awesome! Wish we could sight them here in the big city. But we can sight your fun, enthusiastic blog entries!

  54. Messier Tidy Upper

    @51. Cedric Katesby quoted :

    “Science Correspondent Ronald Bailey who can explain why the Florida coast we are sailing from will still be there a century from now no matter what global warming activists tell you.”

    Well to be fair the coast *will* still be there – it might just be submerged under a bit of water! He never wrote that it would be above the waves then.. ;-)

    I don’t think Cedric Katesby is trolling and I don’t think Matt Welch is doing anything good by writing that & having it associated with the JREF.

    @49. Gary Ansorge :

    … even if all of the Greenland ice melts, it will only raise sea level 6 meters. I don’t think that’s enough to inundate all of Florida.

    Well the thing is, it won’t only be Greenland that’s melting. There’s also the Arctic, the Antartic, the alpine glaciers and the thermal expansion of the water to consider. :-(

  55. AliCali

    @ 54: Tim G. Thank you. You got my point.

    @ 56: Messier Tidy Upper: Careful about what you say regarding Global Warming, as you know how some will jump over any little mistake and ignore all the other factual statements.

    For rising sea level, the Arctic melting shouldn’t have an effect, as it’s floating and already displaces its own weight.

  56. Nick

    Yeah, I always liked it when I have one of those what I call “science moments” where you see something happen and your knowledge of science helps you predict what will happen next. Sure enough, it does. You can impress your friends that way, but the right opportunity has to present itself.

  57. JB of Brisbane

    @AliCali #17 – you mean the Imperial (British) system of weights & measures, don’t you? Although I do take your point about “30,000 feet”; Australia has been “metricated” (or should that be “SI’d”) since 1972, and I still bristle when I hear an altitude expressed in metres. Okay, I suppose the American version of the Imperial system uses smaller gallons and tons, and does away with such measures as chains, furlongs, stones and hundredweights…

  58. Mark Hansen

    Late in, but anyways…
    I’m disappointed in the BA. He knows better than most that real astronomers never spend time just looking at the sky. Just ask any UFO True Believer™. And as for mundane explanations for moving objects in the sky that involve satellites… more proof that he’s in the pocket of Big Gummint™. :)

  59. dcsohl

    Let’s turn to one of my favorite sites, Wolfram|Alpha. According to WA, the ISS orbits at an average altitude of 226.1 miles, at an average velocity of 17,214 mph. Phil’s satellite, Cosmos 2219, orbits at an average altitude of 533.5 miles, and an average velocity of 16,614 mph.

    In other words, Cosmos 2219 is moving at 96.5% the velocity of the ISS.

    So I hereby declare AliCali and kuhnigget to be the winners of the thread. Phil did not notice a 3.5% speed difference. He mostly noticed that the satellite was 2.3 times as high as the ISS (and thus, taking 2.3 times as long to cover the same angular difference from his point of view).

  60. Teshi

    I just woke up feeling a bit down and read this. Thanks.

  61. boygenius

    July of ’09 I was at a music festival in Oregon. Some stargazer or another had informed the band that the ISS with the space shuttle docked would be passing over during their set, and had calculated the exact time that it would be visible. The band stopped playing mid-song and had the lights killed so that we could watch it pass over.

    Standing there in that crowd of 5,000 people watching that little point of light arc across the sky, in almost complete silence and with zero light pollution, was one of the most intense experiences I’ve ever had.

    Even if I hadn’t been high on mescaline, it would have been one of those transcendent moments.

    Once the little point of light disappeared behind the tree-line, the lights came back up and the band picked up where they left off. There was some crazy energy flowing through that place for the remainder of the night.

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