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View Full Version : Why is the Sun so small on Mars?



Diamond
2004-Feb-27, 08:21 PM
...because its further away. http://news.yahoo.com/news?tmpl=story&u=/ap/20040227/ap_on_sc/mars_rovers_3

I get amazed that I forget that those rovers really are on a different planet looking at the same sun from much further away. The very familiarity of looking at rocks on Mars that are similar to Earth's causes me to forget the perspective of how far away these rovers are from us. :o

man on the moon
2004-Feb-27, 09:12 PM
spirit is also taking pictures of humphrey...better close your curtains mr turtle!

seriously though, i think the eclipse idea is really cool! anyone know how big the moons would appear and how they compare to the sun size?

i know they are smaller, but are they also closer?

stelmosfire
2004-Feb-27, 09:27 PM
This site (http://star.arm.ac.uk/~aac/mars/Information.html) gives some interesting information about Phobos and Deimos as seen from the surface of Mars.

I ran across a reference elsewhere that Phobos eclipses only about one third of the solar disk during a "full" solar eclipse; Deimos, being both smaller and further away, would block out far less.

George
2004-Feb-27, 09:49 PM
seriously though, i think the eclipse idea is really cool! anyone know how big the moons would appear and how they compare to the sun size?

i know they are smaller, but are they also closer?

Phobos is very close to the surface (about 6,000 km)!

It is 27x22x19 km in dimension.

At 27 km, I get .26 arc seconds vs. .35 arc seconds for the sun. So Phobos will not likely come close to a total eclipse but still dim the sun quite a bit.

pelzo63
2004-Feb-28, 09:49 AM
At 27 km, I get .26 arc seconds vs. .35 arc seconds for the sun. So Phobos will not likely come close to a total eclipse but still dim the sun quite a bit.

Could the moons both occult each other at the same time as they partially eclipse the sun? or better yet, could phhobos and deimos both be in front of the sun, but at very slightly different positions(possibly their discs "touching"), and both in front of the sun, creating a solar disc image that is a crescent, with an extra "bite" taken out?

John Kierein
2004-Feb-28, 10:05 AM
Your calculations are way off.
11 arc min for the bigger moon and varying for the sun.

Eroica
2004-Feb-28, 11:25 AM
Your calculations are way off.
11 arc min for the bigger moon and varying for the sun.
Yeah. A quick calculation gives me about 14 arcseconds [see edit] for the Sun and about 10 for Phobos.

[Edit: that's arcminutes, not arcseconds! Thanks milli360.]

milli360
2004-Feb-28, 01:29 PM
Your calculations are way off.
11 arc min for the bigger moon and varying for the sun.
Yeah. A quick calculation gives me about 14 arcseconds for the Sun and about 10 for Phobos.
You meant arcminutes, right?

Mars is only 1.5 AU from the Sun (last August it was as close as 1.38 AU), so the Sun's diameter should appear to be about 1/1.5 that it appears from Earth, which is a tad over 30 arcminutes. So, on Mars it should have a diameter of about 20 arcminutes. It's not that much smaller.

eburacum45
2004-Feb-28, 02:20 PM
Or you can look at it like this;
http://www.forteantimes.com/forum/attachment.php?s=2e6980f41643d1eea09a26363fbd9c46& postid=344620
(thanks to Celestia)

Eroica
2004-Feb-28, 05:45 PM
Your calculations are way off.
11 arc min for the bigger moon and varying for the sun.
Yeah. A quick calculation gives me about 14 arcseconds for the Sun and about 10 for Phobos.
You meant arcminutes, right?

Yeah, I meant arcminutes! ](*,)

George
2004-Feb-28, 07:27 PM
Your calculations are way off.
11 arc min for the bigger moon and varying for the sun.

Oops, thanks. I meant degrees not arc seconds.

Even using the mean diameter of Phobos (22.2km) and 5,978 km distance to from the surface of Mars, I get .21 arcdegrees (12.8 arcminutes) using arctangent. Using 1.39m km for Sun's dia. and 228m km for Mars orbit yields .35 arc degrees (21 arcminutes). So what am I doing wrong? #-o

George
2004-Feb-28, 07:29 PM
Could the moons both occult each other at the same time as they partially eclipse the sun? or better yet, could phhobos and deimos both be in front of the sun, but at very slightly different positions(possibly their discs "touching"), and both in front of the sun, creating a solar disc image that is a crescent, with an extra "bite" taken out?

:-$ .....there are astrologers lurking.

George
2004-Feb-28, 07:30 PM
Yeah, I meant arcminutes! ](*,)

Yah big dummy! :lol:

milli360
2004-Feb-28, 07:37 PM
Your calculations are way off.
11 arc min for the bigger moon and varying for the sun.

Oops, thanks. I meant degrees not arc seconds.

Even using the mean diameter of Phobos (22.2km) and 5,978 km distance to from the surface of Mars, I get .21 arcdegrees (12.8 arcminutes) using arctangent. Using 1.39m km for Sun's dia. and 228m km for Mars orbit yields .35 arc degrees (21 arcminutes). So what am I doing wrong?
About 21 arcmin is what I got for the Sun (see above), and eburacum45's graphic seems to show Phobos about half, so that fits too. John said 11 arcmin, in the quote, instead of your 12.8 arcmin. That's still ballpark.

George
2004-Feb-29, 03:57 AM
About 21 arcmin is what I got for the Sun (see above), and eburacum45's graphic seems to show Phobos about half, so that fits too. John said 11 arcmin, in the quote, instead of your 12.8 arcmin. That's still ballpark.

Thanks. I'm still rusty with my math and hoped I was close.

SpaceTrekkie
2004-Feb-29, 04:18 AM
...because its further away. http://news.yahoo.com/news?tmpl=story&u=/ap/20040227/ap_on_sc/mars_rovers_3

I get amazed that I forget that those rovers really are on a different planet looking at the same sun from much further away. The very familiarity of looking at rocks on Mars that are similar to Earth's causes me to forget the perspective of how far away these rovers are from us. :o

Wow, that picture was like a reality check (altho it makes it all seem less real) it really is sooo far away!

-ST

Eroica
2004-Feb-29, 10:15 AM
Yeah, I meant arcminutes! ](*,)

Yah big dummy! :lol:
Right back at ya! :D


Oops, thanks. I meant degrees not arc seconds.

milli360
2004-Feb-29, 11:49 AM
Wow, that picture was like a reality check (altho it makes it all seem less real) it really is sooo far away!
The sun is small in that photo because of the lens, not because it is so much smaller when viewed from Mars.

George
2004-Feb-29, 07:14 PM
Yeah, I meant arcminutes! ](*,)

Yah big dummy! :lol:
Right back at ya! :D


Oops, thanks. I meant degrees not arc seconds.

Oh so true! :roll: I suppose the error is just a matter of degree, though! :wink:

yaohua2000
2004-Mar-02, 12:39 AM
Or you can look at it like this;
http://www.forteantimes.com/forum/attachment.php?s=2e6980f41643d1eea09a26363fbd9c46& postid=344620
(thanks to Celestia)

I found no transits at Gusev Crater or Meridiani Planum at 22:00:22 on March 3rd, 2004 as shown above.

But I found such a transit at Meridiani Planum this week:

Phobos transit from the Sun on 2004-Mar-06 19:22:03 UTC persisted for ?? seconds.

I computed the above with Horizons On-Line Ephemeris System introduced by JPL.

George
2004-Mar-02, 01:32 AM
Phobos transit from the Sun on 2004-Mar-06 19:22:03 UTC persisted for ?? seconds.

I'm gonna guess about 22 seconds max. just for grins.

[Phobos @ 47arcsec/sec as seen from center of Mars, 15 arcsec/sec rotation speed of Mars, net of 60 arcsec/sec from surface due to distances and rotation, 21arcmin of Sun]

[I'll be amazed if I'm right]

yaohua2000
2004-Mar-02, 05:11 AM
I'm gonna guess about 22 seconds max. just for grins.

You were correct. :)

I computed the transit again carefully. The transit will start at 19:21:52.4 UT and end at 19:22:14.4 UT, last 22.0 seconds.

At 19:22:03, March 6th, 2004 UT in SCET, the angular separation between the Sun and the Phobos is 0.1999 degrees.

Eroica
2004-Mar-02, 11:49 AM
I don't know what you guys are up to. I can't find any of these transits. Perhaps I have the wrong co-ordinates for the landing sites:

Opportunity: Meridiani Planum, 2 South, 6 West

Spirit: Gusev Crater, 15 South, 185 West

George
2004-Mar-02, 01:38 PM
I'm gonna guess about 22 seconds max. just for grins.

You were correct. :)

My battin average just about doubled. 8) :)

eburacum45
2004-Mar-02, 04:04 PM
Hey- I'm sorry, but the image I captured from celestia wasn't taken from the landing site- it was just a picture taken from behind Phobos, of the Sun, at the distance of the surface of Mars, to show the relative sizes of the Sun and Phobos; that was why I called it a 'mock-up'... apologies if I misled anyone;
now I'm going to try out the landing sites top see what happens at the times you mention. Not sure if Celestia is quite that accurate, though... the shadow of Phobos is small, and Mars is reasonably big...

eburacum45
2004-Mar-02, 05:43 PM
Well, Ive changed the image now to show the transit at 19.52.02 UTC
as seen from the surface of Mars;

this doesn't look like it is anywhere near Opportunity, though-
(damn- this is more difficult than I thought)

yaohua2000
2004-Mar-02, 11:46 PM
I don't know what you guys are up to. I can't find any of these transits. Perhaps I have the wrong co-ordinates for the landing sites:

Opportunity: Meridiani Planum, 2 South, 6 West

Spirit: Gusev Crater, 15 South, 185 West

I computed the above with Horizons On-Line Ephemeris System introduced by JPL.

You can telnet to ssd.jpl.nasa.gov at the port 6775 to use the system.

If you are using Windows, perhaps you have to set up the width and the height of your console manually first, by enter command tty as "tty <lines> <columns>", it no necessary for xterm or gnome-terminal or Linux console.

And then, you can enter "401" to set the target to Phobos, "401" is the code for Phobos.

Enter "E" for Ephemeris, and then "o" for observer. It will ask you the observed site then, enter "c@499" for custom site at "499", the code of Mars.

It will then ask you the coordinate of the custom site, enter "g" and "-354.06,-1.98,0" for 5.94 degrees west, 1.98 degrees south, 0 kilometres in elevation.

It will then ask you the date and time, enter the start date/time as "2004-Mar-06 19:21:30" and the end date/time as "2004-Mar-06 19:22:30", enter the interval as "60" to split the time above as 60 parts.

At last, enter "y" to accept the default settings, and enter "13,23" to set up what should be computed. We want the angle of Sun-Observed-Target, and the angular diameter of Phobos, so enter "13,23".

After a while, you can get a table for the result.

Have fun. :P

Kebsis
2004-Mar-03, 12:11 AM
Have the Mars rovers taken any pictures of the moons of mars yet?

And what was the ultimate fate of Pathfinder?

Eroica
2004-Mar-03, 12:35 PM
I'm absolutely certain that there is no transit of Phobos as seen from Meridiani Planum on 6 March 2004 at 19.22 UT. In fact, at that time, it's dark at the Meridiani Planum!

yaohua2000
2004-Mar-03, 12:47 PM
I'm absolutely certain that there is no transit of Phobos as seen from Meridiani Planum on 6 March 2004 at 19.22 UT. In fact, at that time, it's dark at the Meridiani Planum!

:oops:

It seems you were correct....

I forgot to check if day or night.

But if Mars is a transparent planet, the transit will happen then. :lol: