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tony ennis
05-16-2009, 04:17 PM
Amazing... (http://www.skynyx.fr/legault/atlantis_hst_transit.html)

rockrat
05-16-2009, 07:29 PM
Thats a fantastic photo. Strange that this is the "first" one ever taken. With space travel becoming the norm instead of rare, one would think that an armature had already done this.

Very cool

rock~

Evan
05-17-2009, 01:56 AM
The difficulty involved in successfully obtaining such a photo is hard to explain. Everything has to be just right. The size of the zone of visible transit on the ground is very small, and the orbital parameters of any low earth orbit satellite vary every orbit and must be updated daily at the least.

I haven't checked but I suspect that the margin of error for Hubble's orbit may exceed the size of the zone in which the transit is visible. The problem is mainly due to the irregularity of Earth's gravitational field and the fact that on every orbit Hubble passes over a different part of the surface. Other factors are orbital decay that varies due to the orientation of the telescope as well as light pressure which also perturbs the orbit. Even the orientation of the spacecraft in respect of the gravity field will alter the orbital velocity because of gravitational tides.

Gravitation anomaly map
http://apod.nasa.gov/apod/image/0307/gravityearth2_grace.gif


There is also the matter of time dilation caused by the orbital velocity relative to the Earth's surface. This is counteracted by the fact that the spacecraft is further out of the gravity well than the surface and the net term is that time on the Hubble is slightly faster than on the surface. The difference in "real" vs apparent position is surprisingly large. If not accounted for the cumulative relativistic error of a GPS satellite would be about 10 kilometres actual position per day.

Then we have the ordinary but very important issues such as weather and availability of a suitable observing location at the right time and place. Those two items alone will make it very unlikely that anybody with the appropriate equipment is situated where an observation can be made.

On top of that, it is no small feat to have everything work exactly correctly in the tiny window of time available to take the images.

Hat's off to the fellow that did it.

Ken_Shea
05-17-2009, 02:44 AM
That is just awesome!

Why are planets spherical? I realize they are not ball bearing smooth or round but why are they all essentially spherical?

barts
05-17-2009, 03:40 AM
That is just awesome!

Why are planets spherical? I realize they are not ball bearing smooth or round but why are they all essentially spherical?

Gravity pulls a molten planet into a sphere; that's the lowest energy configuration.

- Bart

macona
05-17-2009, 04:57 AM
That and surface tension. The earth for the most part is still a molten ball of rock.

Evan
05-17-2009, 06:22 AM
Surface tension is a microsopic effect.


Gravity pulls a molten planet into a sphere; that's the lowest energy configuration.


The interesting thing about gravity is that it is the weakest force of the 4 known forces. It is about 10^39 times weaker than the strong force that hold atoms together. However, it is gravity that wins in the end.

Planets are approximately round because even solid granite cannot resist the force of gravity once an object becomes larger than the asteroid Ceres which is a little under 1000 km in diameter.

http://www.planetary.org/explore/topics/asteroids_and_comets/ceres.html

This is also why the tallest mountain known is not on Earth but on Mars instead. Olympus Mons make Everest seem like a foothill. It is about three times higher at around 17 miles tall. This is possible because of the weaker surface gravity of Mars.

John Stevenson
05-17-2009, 06:34 AM
This is also why the tallest mountain known is not on Earth but on Mars instead. Olympus Mons make Everest seem like a foothill. It is about three times higher at around 17 miles tall. This is possible because of the weaker surface gravity of Mars.

Now tell us you walked up that both way when you were a lad, with no shoes on and a slice of bread and dripping :rolleyes:

.

Evan
05-17-2009, 06:49 AM
Well, I didn't wear shoes most of the summer as I grew up in California and I rode my bicycle to school clocking over 1000 miles per semester. It was uphill both ways as I had to cross over a ridge on the way. The steepness of that ridge road is suggested by the name of the road, Hillgrade Rd.

:D:D

John Stevenson
05-17-2009, 06:56 AM
But what about the slice of bread and dripping ?? :rolleyes: :rolleyes:

rockrat
05-17-2009, 08:52 AM
The difficulty involved in successfully obtaining such a photo is hard to explain. Everything has to be just right.

Oh, dont get me wrong. I can imagine the need to be in the right place at the right time. My thought was more toward the amateur. I have seen the amateurs with the The CAS (http://www.the-cas.org) up at Perkins Observatory (http://www.perkins-observatory.org) (shameless plugs) lock up on the shuttle, iss and the such with a dob by hand as well as program the encoders on the more expensive scopes for tracking and photography of the faster moving objects.

It just doesnt seem that far off to take this photo with a solar filter (or hydrogen alpha, that could be sweet) on the scope. And yes, our location does not seem to allow us the best transits, clear skys or seeing but every once in a while even we get lucky.

So couple all of the with groups that chase things such as the solar eclipse, it was just hard to believe that this was the first shot. But hey, some one had to do it first so congrats to Thierry.

rock~

aostling
05-17-2009, 09:33 AM
The steepness of that ridge road is suggested by the name of the road, Hillgrade Rd.
:D:D

There is a Hillgrade Ave in Alamo http://maps.google.com/maps?f=q&source=s_q&hl=en&q=Hillgrade+Ave,+Alamo,+Contra+Costa,+California+9 4507&sll=37.749956,-122.159454&sspn=0.012199,0.013926&ie=UTF8&cd=1&geocode=Fc3TQQId4bi5-A&split=0&ll=37.868485,-122.046261&spn=0.048717,0.055704&t=h&z=14&iwloc=A. Was that near your house?

That was long before I-680 existed, and I expect you felt like you were living on the outer frontier of the East Bay.

Evan
05-17-2009, 09:46 AM
Did you notice how long the transit lasted? 0.8 seconds. The footprint of the visible zone was around 6 kilometers. Locking up on the shuttle isn't hard when you only have to be concerned about getting JUST it in your field of view. The problem here is being able to forecast long enough in advance where to set up in order to capture the event so that your field of view also includes the sun. It's like lighting a match with a .22 halfway to the target AND hitting the bull at the same time.

tony ennis
05-17-2009, 10:07 AM
But what about the slice of bread and dripping


Luxury! When I was growing up we had to farm our own bread while we were walking to school! And we didn't have shoes! AND WE LIKED IT THAT WAY!

Evan
05-17-2009, 10:14 AM
Well, that took me by surprise. They have street level view of the entire area including right in front of our old home. I lived just down Lunada Lane a short distance from Hillgrade. Here is the view at the intersection looking up Hillgrade. Amazing technology.

http://ixian.ca/pics6/hillgrade.jpg

Back then it was 1/10 as many houses as there are now with large areas in walnut orchards and just open fields. It was excellent for sky watching and on one occasion we had taken my father's six inch telescope to the top of Mt. Diablo and discovered the comet Ikeya Seki on the same night as the official discovery. We knew nothing about reporting it but if we had the comet would have been named Ikeya-Seki-Williams.

dp
05-17-2009, 11:58 AM
Interesting technology - here's the house in Berkeley I lived in from 1960 until 1964. Dad is still there: http://tinyurl.com/r7f6ju - I'd better remind him to put away his trash cans :)

aostling
05-17-2009, 02:15 PM
Well, that took me by surprise. They have street level view of the entire area including right in front of our old home. I lived just down Lunada Lane a short distance from Hillgrade.

Here is the Zillow link, showing how much those houses are estimated to be worth today: http://www.zillow.com/homes/map/alama,-ca_rb/#/homes/for_sale/map/alama,-ca_rb/37.8684,-122.045581,37.866321,-122.048199_rect/17_zm/. You can click on address links to bring up the history of any house, including the year it was built.

rockrat
05-17-2009, 06:07 PM
Did you notice how long the transit lasted? 0.8 seconds.

Fair enough, that was a tricky one no doubt.

rock~

Evan
05-17-2009, 06:22 PM
Here is the Zillow link, showing how much those houses are estimated to be worth today

1.18 million dollars, estimated monthly payment $5000. That's just nuts. I happen to know how much my father paid for it in 1957, $25,000. That means it has appreciated about 47 times since then. Who can afford a 5 grand monthly payment?

andy_b
05-18-2009, 09:48 AM
There is also the matter of time dilation caused by the orbital velocity relative to the Earth's surface. This is counteracted by the fact that the spacecraft is further out of the gravity well than the surface and the net term is that time on the Hubble is slightly faster than on the surface. The difference in "real" vs apparent position is surprisingly large. If not accounted for the cumulative relativistic error of a GPS satellite would be about 10 kilometres actual position per day.



are you saying time dilation comes into play due the orbital corrections and inaccuracy in predicting where the shuttle will be, or due to it somehow affecting the photographic process itself?

andy b.

Evan
05-18-2009, 11:36 AM
Time dilation definitely plays a part in determining Shuttle position in advance. The transit time for this particular observation was only 800 milliseconds. The Shuttle is in a much lower orbit than the GPS satellites and consequently orbits at a much higher velocity. This changes the effect since the shuttle is deeper in the gravity well but moving faster. I don't know what that works out to but I doubt it balances. It all comes down to the concept of simultaneity. We can never say that two events occur at the same time in two different places. It takes time for any type of communication to travel from a to b so we can only predict what the time is elsewhere, not actually measure it.

The error in the GPS clocks is a gain of around 38,000 nanoseconds per day. That is compensated by setting the clocks to run slow before launch. Fine corrections are determined by observation during the life of the satellites and corrections are broadcast to every GPS receiver which then use those relativistic correction to adjust the reported positions of the satellites for triangulation.

The light by which the shuttle is seen in the Hubble orbit takes about 2 milliseconds to reach Earth so that alone produces an apparent error in position of 50 feet unrelated to time dilation.

The relativistic corrections are not a big factor since they can be easily predicted and corrections applied. The main factor is the varying influence of the Earth's gravity and the effect it has on orbital velocity. That is a non-relativistic problem and understanding and correcting for it requires a detailed knowledge of the variations in local gravity as shown it the animated world gravity map above. Because the number of terms to be considered expands exponentially with time orbital coordinate predictions must be updated frequently. With current technology extremely accurate predictions can be made for up to several orbits up to an altitude of around 6000 km. Above that only close approximations are possible.

It isn't well known that the main reason that interplanetary space craft almost always have a camera onboard regardless of their mission is that the camera is essential for navigation. The problems of predicting actual location based on first principles are not solvable with our current understanding of mathematics. We don't have an analytic solution to the three body orbital parameters problem so spacecraft navigation is carried out primarily by using Newtonian physics and then "looking out the window" to see where you really are. This applies to all missions beyond Earth orbit.

Earth orbit orbital parameters are determined by actual observation of the spacecraft combined with observation from the spacecraft. These are updated regularly with the frequency depending on the particular satellite and it's mission.

The Shuttle of course is tracked continuously so that corrections may be made after it manoeuvres or even just changes attitude since the flight attitude affects the atmospheric drag the Shuttle experiences.

In all, NASA has a very good idea where the shuttle will be in a few hours time but that confidence level drops quickly with time. Orbital element normally available to the public are published daily that that may not be good enough to determine where to set up in advance to take a picture such as featured in this thread. There is a large element of good luck involved.

macona
05-19-2009, 01:38 AM
I suggest everyone go back to the link in the first post. The guy did it two more times!

dp
05-19-2009, 02:11 AM
I think time dilation in a near spherical orbit is not much of a factor. In eliptical orbits it becomes more important for predicting brief events such as star occlusion by small bodies. But gravitational anomalies in heavy orbiting objects are tempered by inertia of the object. It's not like they're bouncing around like soccer balls in the surf.

Years ago an acquaintance of mine from Hawaii, astronomer Dr. David Tholen, predicted the shadow of Charon, one of the moons of Pluto, would be cast when it occluded an unremarkable star, and that the shadow, stretching billions of miles across space, would drag it's fleeting finger across the earth's surface, and it did. Now that's pretty good math. From that event it was possible to determine the size and mass of Charon.

Dr. Tholen is the fellow who took the picture of comet Hale-Bopp that was made famous for it's alleged "companion" object by radio personality Art Bell: http://www.ifa.hawaii.edu/images/hale-bopp/tholen-sep1/

In the mid 1980's I was in Anchorage, Alaska during a full moon and was attempting to catch passenger jets transiting the moon. The moon is close to the horizon much of the time in Alaska. I was using a long lens so the moon filled the viewfinder, but as with the shuttle and the sun, it takes just the right kind of luck. I did get the shot. It was a Boeing 737 - not a large bird, but it was caught beautifully in perfect turning ascent profile, black against the brilliance of the moon. I sold it to an ad agency for $800.

Evan
05-19-2009, 05:57 AM
But gravitational anomalies in heavy orbiting objects are tempered by inertia of the object. It's not like they're bouncing around like soccer balls in the surf.


They aren't "bouncing around" but how long does it take for a weight swinging around your head to respond to cutting the rope?

The orbital parameters change at the propagation velocity of gravity or with the curvature of space in the vicinity of Earth. Orbital velocity tracks these variations exactly to a measured precision of about 10^-14.

Forrest Addy
05-19-2009, 06:47 AM
Evan, Just as a nitpicky point of concern wouldn't the local gravitation affect the orbit radius and not the speed? Well the speed too, subject to the object's inertia, orbit eccentricity, and orital energy but as a algebraic sum of the function of radial distance. Interesting physics problem good for an hour's computation on a dozen coffee shop napkins and later cheerful arguement with the (wanna-be) orbital mechanics cognoscenti.

Anyway, In photography we have an expression intended to deflect critical comment when we're duffering about without a clue: it's called "brackeing." - in this case multiple exposures as rapid as the camera will take them in sequence. One of the frames has gotta contain the image of transit.Kinda like shotguns and hand grenades.

Lessee. The sun is about 1/2 degree in apparent diameter so the width of a transit's track on the earth would be about 1/2 degree of latitude, wouldn't it? At about 70 statute miles per degree that's 210 miles (minus a pinch just to ensure the image is on the sun's disk) I suppose the average back yard astronemer can find the orbital elements of the transiting object and be at the place and time far enough in advance to set up. Finding the target is easy. Look for the sun through the viewfinder. When your retina turns to french toast, engage the polar motion and start the cemera.

Next question is: where's the sunspots? Must be a minimum or something.

Evan
05-19-2009, 07:44 AM
Orbital velocity is directly tied to orbital radius. You can't change one with out the other changing. When the satellite passes over an area of lower gravity it's like letting the rope out on the weight you are swinging. It swings slower and takes longer to come around full circle. Of course the opposite is true when you pull the rope in , or the satellite because it is over an area of stronger gravity.


The sun is about 1/2 degree in apparent diameter so the width of a transit's track on the earth would be about 1/2 degree of latitude, wouldn't it? At about 70 statute miles per degree that's 210 miles (minus a pinch just to ensure the image is on the sun's disk) I suppose the average back yard astronemer can find the orbital elements of the transiting object and be at the place and time far enough in advance to set up. Finding the target is easy. Look for the sun through the viewfinder. When your retina turns to french toast, engage the polar motion and start the cemera.



The size of the zone in which the transit was visible was only about 5.6 kilometres. It's the geometry at work. Because the Shuttle is so comparatively close to Earth as you move even a little from the occultation zone you are viewing the shuttle at a rapidly increasing side angle that doesn't line up with the sun. Your calculation makes the assumption that the angle of view to the object doesn't change much as you move, such as is the case with the Moon.

dp
05-19-2009, 12:50 PM
The GRACE tandem satellite mission was launched specifically to study these gravity variations, and some of the data are presented here:

http://www.csr.utexas.edu/grace/operations/configuration.html

It can be seen that the range variations between the two satellites are there and can be seen in the data, and are also very predictable in the long term. Short term is quite less so because each satellite is in a different orbit and subject to different gravitational affects on orbital period. And over short periods the change is quite small as is the rate of change.

Getting back to the shot of the shuttle against the sun - the photographer captured the entire sun in his photos which ensured if a sun transit occurred he could capture it. He also bracketed the shots as Forrest suggested, shooting several frames in rapid sequence. Also interesting in the two shots a saw a few days ago is how much the angular orientation of the shuttle changed in the short span of .8 seconds as it did the transit. Makes me wonder if they were shot at different times of day? I've not seen any info on that.

Evan
05-19-2009, 04:18 PM
The animated map I posted is from the Grace data. What they don't mention on that page you linked is that the Grace satellites, which are in a polar orbit in order to study the entire Earth, are the least affected when in a polar orbit. A polar orbit automatically averages out the major effects every rotation of the Earth since the satellite(s) pass over all regions an equal number of times and in both directions. The Grace mission is designed so that the variations of local gravity will average out of the two satellites orbits. They are measuring the differential in orbital velocity over a mere 137 miles of orbital separation at any time which is only about 1.6% of an orbit.

That means that the changes they are able to measure would be exaggerated by as much a 50 to 60 times in just one orbit when the orbit passes over areas with only one large anomaly such as a continental mountain range or a major ocean depression.


The orbits to which the shuttle is restricted see much larger variations and each subsequent orbit is over different territory than the last. The shuttle orbit may not pass over the same exact region twice in the same mission depending on the orbital inclination and altitude.

As to being in the right place at the right time, just think about the lengths people must go to to capture an eclipse of the sun. And yet they are much easier to predict, the ground track is far larger because of more favorable geometry and people have years of advance notice. If you were to throw a dart at a large wall map of the western hemisphere how often do you think it would end up in a handy place to set up and view a shuttle transit of the sun, keeping in mind that the occultation zone is about the size of the point on the dart?

I found a paper that is open text and deals with precisely this subject. After reading it it is surprising that the pictures were even taken from the right state let alone inside a 6km circle. The variations possible are huge, amounting in some cases to single orbit changes of degrees of angle and minutes of time.

http://www.geophysics.ut.ac.ir/JournalData/1386A/Eshagh.pdf

dp
05-19-2009, 06:35 PM
Add to all that the possible need for the shuttle to dodge chunks of debris - the current orbit is a a more dirty place that the ISS orbit, and that is quite an unpredictable event of unknowable magnitude. I wonder how long the track was on earth from the shuttle's shadow.

Evan
05-19-2009, 07:36 PM
I wonder how long the track was on earth from the shuttle's shadow.


Nearly the entire time the Shuttle is on the day side. However, for most of that track the slant range to the shuttle will be so long that it won't be resolvable by any portable telescope. Not only does the slant range increase tangentially with viewing angle so does the atmospheric refraction to add up to .5 degree of apparent position error near the horizon.