APOD: Opposite the Sun (2010 Sep 18)

APOD Robot · Post by **APOD Robot** » Sat Sep 18, 2010 4:00 am

Opposite the Sun

Explanation: Chances are the brightest star you've seen lately is actually planet Jupiter. Jupiter rules the sky in this labeled view of a starry September night from the Alborz mountains in Iran, complete with the trail of a red flashlight illuminating the mountain road. On September 21st (Universal Time) Jupiter will be at opposition, the point opposite the Sun along its orbit, rising just as the Sun sets. For this opposition, Jupiter will be slightly brighter and closer to planet Earth than in any year since 1963. Much fainter and also approaching its own opposition on September 21st, is the distant planet Uranus. Very near Jupiter on the sky, the fainter planet is easy to spot in binoculars (similar to the inset view), well above and right of brilliant Jupiter and about as bright as one of Jupiter's own Galilean moons. Remarkably close to the opposition of both planets, the point on the sky exactly opposite the Sun on September 23rd is marked the Vernal Equinox. On that date, a Full Moon will join the celestial scene. Of course, any Full Moon is also at opposition.

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owlice · Post by **owlice** » Sat Sep 18, 2010 7:41 am

I just went out to look at Jupiter, hanging high overhead; it's so bright, my eyes put a halo around it, as they do any bright point of light when it's dark out.

Today's image first appeared on the Asterisk here: http://asterisk.apod.com/vie ... 29&t=21104

Included on the thread is a cute, very short movie that shows... well, you'll see!

Ann · Post by **Ann** » Sat Sep 18, 2010 11:10 am

Beautiful picture. Amazing conjunction. Wonderful skyscape. Delightful star colors. Beautiful "interplay" between Earth and sky. Lovely red double trail caused by the observer carrying a red lamp. Perfect inset with close-up of Jupiter and Uranus.

In short, that's a great picture and a lovely APOD!

Ann

orin stepanek · Post by **orin stepanek** » Sat Sep 18, 2010 12:21 pm

So we have Earth; Jupiter; and Uranus lined up.

neufer · Post by **neufer** » Sat Sep 18, 2010 12:36 pm

Code: Select all

moon/     angular  Magnitude
planet     size
-----------------------------------------
Ganymede   1.9"      4.61    1610
Io         1.3"      5.02    1610
Europa     1.1"      5.29    1610
.......................................
Uranus     4.1"      5.32    1781
.......................................
Callisto   1.7"      5.65    1610
Titan      0.9"      8.20    1655

---------------------------------------------------------------
Uranus is visible to the naked eye, has a fair amount of parallax (in just 4 days!) and sits right on the ecliptic where people have recorded the motions of Saturn & Jupiter for millennia. They even recorded the some of Saturn & Jupiter's moons since 1610/1655.

So why did it take until 1781 (171 years after Galileo) to discover Uranus

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___ King Lear > Act I, scene V

Fool: The reason why the seven stars
. are no more than seven is a pretty reason.

KING LEAR: Because they are not eight?

Fool: Yes, indeed: thou wouldst make a good fool.

---------------------------------------------------------------

http://en.wikipedia.org/wiki/Uranus wrote:
<<Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy), but initially reported it (on 26 April 1781) as a "comet". Herschel "engaged in a series of observations on the parallax of the fixed stars", using a telescope of his own design.

He recorded in his journal "In the quartile near ζ Tauri … either Nebulous star or perhaps a comet". On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place". When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:

“ The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed. ”

Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it">>

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mexhunter · Post by **mexhunter** » Sat Sep 18, 2010 1:24 pm

It is a beautiful composition, also has this interesting counterpoint to the red light on the path, as noted Ann.
Grettings
César

Axel · Post by **Axel** » Sat Sep 18, 2010 4:29 pm

Why is the Vernal Point between the two planets? That certainly is not the case today or on the coming autumn equinox. I have doubts about the relative position of the three points for any moment this year or next - unless there's something about the orientation of the photo I don't get.

neufer · Post by **neufer** » Sat Sep 18, 2010 6:10 pm

Axel wrote:
Why is the Vernal Point between the two planets? That certainly is not the case today or on the coming autumn equinox. I have doubts about the relative position of the three points for any moment this year or next - unless there's something about the orientation of the photo I don't get.

Because the ecliptic runs between Uranus & Jupiter.

It is also near to the spot where the Earth is seen from the Sun at Autumnal Equinox:

: Click to view full size image

Axel · Post by **Axel** » Sat Sep 18, 2010 8:45 pm

neufer wrote:
Axel wrote:
Why is the Vernal Point between the two planets? That certainly is not the case today or on the coming autumn equinox. I have doubts about the relative position of the three points for any moment this year or next - unless there's something about the orientation of the photo I don't get.
Because the ecliptic runs between Uranus & Jupiter.

It is also near to the spot where the Earth is seen from the Sun at Autumnal Equinox:

I know that! I just realized the picture must have been taken on 27 August during a brief period when the planets were on opposites sides of 0° ecliptic longitude (and the Moon was there too). The ecliptic must be curved in this image, otherwise I cannot see how both planets could have south latitude and the ecliptic could pass through the zodiacal constellations.

Joe Stieber · Post by **Joe Stieber** » Sun Sep 19, 2010 8:03 am

Axel wrote:
neufer wrote:
Axel wrote:
Why is the Vernal Point between the two planets? That certainly is not the case today or on the coming autumn equinox. I have doubts about the relative position of the three points for any moment this year or next - unless there's something about the orientation of the photo I don't get.
Because the ecliptic runs between Uranus & Jupiter.

It is also near to the spot where the Earth is seen from the Sun at Autumnal Equinox:
I know that! I just realized the picture must have been taken on 27 August during a brief period when the planets were on opposites sides of 0° ecliptic longitude (and the Moon was there too). The ecliptic must be curved in this image, otherwise I cannot see how both planets could have south latitude and the ecliptic could pass through the zodiacal constellations.

Actually... the ecliptic does not run between Jupiter and Uranus in the picture. It's not shown, but the ecliptic runs from the lower left (between Aries and Cetus) through the cross marking the Vernal Equinox and then continues to the upper right below the Circlet of Pisces. The planets themselves are both southwest of the cross (Uranus is a speck at the upper edge of the overexposed blob for brilliant Jupiter in the main picture). Was there some confusion because the cross is between the labels for Jupiter and Uranus?

Furthermore, according to Tafreshi's respective web page, the picture was taken on September 11, 2010 (click "a starry September night" in the APOD text). On that date, both planets had slightly negative ecliptic latitude, approximately 1.6 degrees for Jupiter and 0.8 degrees for Uranus, and they were about 359.7 and 359 degrees ecliptic longitude (not that the longtitude matters too much to the point at hand since for at least +/- 30 days from the 11th, the paths of both planets are running pretty much parallel to the ecliptic). Nothing mysterious about that since their orbital planes are tilted slightly relative to the ecliptic.

Henning Makholm · Post by **Henning Makholm** » Sun Sep 19, 2010 2:46 pm

APOD Robot wrote:On September 21st (Universal Time) Jupiter will be at opposition,

Precision is a good thing. We wouldn't want anyone to misunderstand it, so they went out and looked up on September 21st CEST, for example. They might miss it.

Beyond · Post by **Beyond** » Sun Sep 19, 2010 9:08 pm

From what i could find, september 21st CEST is the last day of it, but it really does not say it anywhere.

Starship Asterisk*

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