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Totally off-topic thread (Talk about anything.)

Totally off-topic thread

midtskogen

Date: Wednesday, 13.08.2014, 18:14 | Message # 2506

Star Engineer

Group: Users

Norway

Messages: 1674

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So how accurate is heavens above? Last night it predicted a -8.4 iridium (94) for me at 00:09:51UTC (az 173 alt 56), and sure enough it did appear on time (±1s):

Close up:

Video

Hard to say whether the brightness of -8.4 is accurate, though.

NIL DIFFICILE VOLENTI

Edited by midtskogen - Wednesday, 13.08.2014, 18:16

Watsisname

Date: Wednesday, 13.08.2014, 23:45 | Message # 2507

Galaxy Architect

Group: Global Moderators

United States

Messages: 2613

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The flare magnitude calculation itself is fairly straightforward, as it's just the distance, geometry, and orientation of the satellite and its panels relative to the observer and light source. However, it is also very sensitive. A few tenths of a degree difference in angle can cause a full magnitude difference in brightness.

Sources of error will be therefore be
(i) Observer location uncertainty (very small, since you're using GPS coordinates).
(ii) Satellite location uncertainty (depends on how old the TLE is).
(iii) Satellite orientation uncertainty (haven't a clue).

I've checked flare intensity against other sites and software and they don't always give the exact same answer, but it's usually within a magnitude, or perhaps two mags at worst. If heavens-above told you -8.4 I'd say give or (more likely take) 1 or 2 mags. Calsky gave you a -7.4 for the same flare, with an angle offset of 0.02 degrees and 0.3km centerline distance. Also, it helpfully warns

"This is a spare satellite or its status is unknown. Brightness estimate may be unreliable and flare time accurate to a few seconds."

midtskogen

Date: Thursday, 14.08.2014, 07:24 | Message # 2508

Star Engineer

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Norway

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Certainly in this case with -8.4 the uncertainty is in the dimmer direction, but if Calsky says -7.4 and 0.3 off the centreline, it's hardly only satellite location and orientation uncertainty, since Calsky says near perfect orientation, yet one full magnitude weaker (I think heavens above even said 1 km from the centreline, but even that is close to perfect orientation). Certainly, Heavens Above and Calsky wouldn't disagree so much about the distance so the difference becomes one magnitude.

Heavens Above sometimes gives magnitudes close to -9. In a really dark, moonless location it would mean that shadows would easily become visible. But I'm not convinced that they ever get that bright.

NIL DIFFICILE VOLENTI

Watsisname

Date: Thursday, 14.08.2014, 14:11 | Message # 2509

Galaxy Architect

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United States

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Oh no, the discrepancy between heavens-above and calsky's prediction is not location/orientation error. I'm saying that discrepancy's given a particular source for magnitude calculation are caused by those. Which source has the correct magnitude for a given pass, I haven't a clue. Heavens-above tends to give the most optimistic estimates for the brightest passes, but I don't know how it compares through the whole magnitude scale. It's usually pretty good for the fainter ones. I'm not convinced Iridium flares ever get quite as bright as -9 either, and I've seen several of the theoretical brightest ones. But it's hard to know without a suitable object to serve as a comparison.

Basically how flare magnitude prediction works is that you do the math and derive a formula for the magnitude as a function of knowable or calculable parameters: apparent angular size of the reflecting region, how much of the light source it is reflecting, and how bright that source is. Albedo is of course going to factor in as well, but we can't know or calculate what it is from first principles. However, we can reasonably expect that all the satellites in the Iridium constellation have roughly the same quality panels. So we have an equation with several free variables, one unknown (albedo), and one output. The next step then is to calibrate the formula -- solving for the albedo -- by plotting predicted outputs against observations. The best fit will uniquely determine the equation you use.

If different sites/softwares presumably did not all produce their formula from common data (I don't know, I've never done that kind of work), they're not going to agree perfectly on a given pass. That's why it's best to use several sources to get an idea of what to expect, if knowing the exact magnitude is important. Or do it yourself with photometry. smile

Tac1017

Date: Thursday, 14.08.2014, 19:33 | Message # 2510

Explorer

Group: Users

United States

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Hi. I am Tac1017, and I hunt earth-like planets right in our milky way. My favorite thing to do in space engine is to research my terra planets an study them, and make up the species that live on them! As of now, I found over 10 of these planets, and we could live on each one if we could go to them.

Btw, about my age, I tried to make an account with my real bday, but it couldn't work, and I had tons of recommendations like special filters, multiverses, and even alien life detections like night side lights on other planets with it. I am devoted to the game since I first got it a few months back, and I want to blend in with the others into this game.

Added (14.08.2014, 18:33)
---------------------------------------------
Yes, even though I am 13, I am very intelligent and mature, so you can trust me smile

The Terra Hunter of the Milky Way!

(By the way, I was born in 2001, NOT 1972 XD)

Antza2

Date: Thursday, 14.08.2014, 19:45 | Message # 2511

World Builder

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Finland

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Tac1017, Your post doesn't need it's own thread. Please read the forum rules before posting.

Go to antza2.deviantart.com for cool photos!

Salvo

Date: Friday, 15.08.2014, 09:10 | Message # 2512

Star Engineer

Group: Local Moderators

Italy

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Tac1017, hi to you too!

Quote Antza2 (

)

Your post doesn't need it's own thread. Please read the forum rules before posting.

Maybe it fits better in "Hello - From New Members" thread smile

The universe is not required to be in perfect harmony with human ambition.

CPU: Intel Core i7 4770 GPU: ASUS Radeon R9 270 RAM: 8 GBs

(still don't know why everyone is doing this...)

midtskogen

Date: Sunday, 17.08.2014, 14:32 | Message # 2513

Star Engineer

Group: Users

Norway

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Woke up 5am GMT+0200 yesterday and got to bed 22:30 GMT-0700. Woke up again 01:30 GMT-0700 (my body thought it was 10:30) and haven't fallen asleep again since. It's now 06:30 and I give up. My brain is not going to work today.

NIL DIFFICILE VOLENTI

spacer

Date: Sunday, 17.08.2014, 15:05 | Message # 2514