Starship Asterisk*

Vesta Near Opposition

Explanation: Main belt asteroid 4 Vesta is at its brightest now. The small world is near opposition (opposite the Sun in the sky) and closest to Earth. But even at its brightest, Vesta is just too faint to spot with the naked-eye. Still, over the next few days it will be relatively easy to find in the constellation Leo, sharing a typical binocular field of view with bright star Gamma Leonis (aka Algieba). In fact on February 16 Vesta passed between Gamma Leonis and close neighbor on the sky 40 Leonis. Gamma Leonis is the brightest star in these two panels, while the second brightest star, 40 Leonis, is directy to its right. As marked, Vesta is the third brightest "star" in the field. Vesta shifts position between the two panels from well below 40 Leonis on Feb. 14 to near the top of the frame from Feb. 16, shooting the gap between the close Gamma/40 Leonis pair. Of course, premier close-up views of the asteroid will be possible after the ion-powered Dawn spacecraft arrives at Vesta in August of 2011.

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I'm new to the forum, so I beg your pardon if this type of question has been asked to death, but why do the background stars in the two shots exhibit a color change?

The small world is near opposition (opposite the Sun in the sky) and closest to Earth.

I don't understand this, how can something be in opposition and still be closest? Unless one or both orbits are extremely elliptical of course, but that's not the case here I think.

inertnet wrote:

The small world is near opposition (opposite the Sun in the sky) and closest to Earth.

I don't understand this, how can something be in opposition and still be closest? Unless one or both orbits are extremely elliptical of course, but that's not the case here I think.

Opposition means that the earth is between the sun and Vesta.
Conjunction means that the sun is between the earth and Vesta.

Assuming that the distance between the sun and Vesta is roughly constant
then the earth is 2 AU closer to Vesta during opposition than conjunction.

yasgur wrote:I'm new to the forum, so I beg your pardon if this type of question has been asked to death, but why do the background stars in the two shots exhibit a color change?

At a guess, I'd say the Feb 16 photo is slightly blurred due to atmospheric conditions.

yasgur wrote:I'm new to the forum, so I beg your pardon if this type of question has been asked to death, but why do the background stars in the two shots exhibit a color change?

Without more image details, it's impossible to say for sure. The appearance of the image makes me think it was taken with a color camera of some sort; such cameras usually produce notoriously poor color accuracy on astronomical targets. I would speculate that the Feb 16 image was made under poorer sky conditions than the Feb 14 image- thin haze, perhaps. This would require applying different contrast curves to the images in order to approximately balance the star intensities, changing the appearance of the stellar profiles and also creating color balance problems.

Another possibility is that the two images were made at very different times. This object is at opposition, so it is visible in the sky all night. If one shot was made when it was near the horizon, and the other when it was near the zenith, atmospheric extinction effects could produce a very different appearance between the two.

Thanks for the replies. I didn't think of something as simple as haze or the timing of the shots.

Opposition means that the earth is between the sun and Vesta.
Conjunction means that the sun is between the earth and Vesta.

Is there a word that describes the situation when/if an object (other than the moon, Venus or Mercury) is between the sun and the earth?

After many months of visiting APOD and its associated Digg page where I asked (usually unanswered) questions, I finally realised that this bb is here - I really appreciate you people that take the time to answer my questions!!

Neufer's definitions of conjunction and opposition are for any superior object (objects with orbits further from the sun than Earth). For inferior objects (objects with orbits nearer to the sun, e.g. Venus and Mercury), the terms inferior conjunction (object between Earth and Sun) and superior conjunction (Sun between Earth and object) apply. Objects that are in orbit around the Earth (e.g. the Moon) are in opposition (e.g. Full Moon) when the Earth is between the object and the Sun, and in conjunction (e.g. New Moon) when the object is between the Earth and the Sun.


Wikipedia Commons: Figure for Positional Astronomy

biddie67 wrote:Is there a word that describes the situation when/if an object (other than the moon, Venus or Mercury) is between the sun and the earth?

Conjunction is fine for that, as well. In its most general sense, two objects are in conjunction if they appear close together in the sky. Assuming your object is in orbit around the Sun, when it lies between the Earth and Sun that is called inferior conjunction. When it is on the other side of its orbit, so that the Sun is between it and the Earth, that is called superior conjunction. Obviously, only objects in orbits smaller than Earth's have inferior and superior conjunctions. Objects in larger orbits are in conjunction when they are on the opposite side of the Sun (from the Earth) and in opposition when they are on the opposite side of the Earth (from the Sun).

Feb 19th image - 46 subs at 200 seconds per sub. Brightest star to the left is Algieba.



Greg

Greg, That's awesome.

Opposition means that the earth is between the sun and Vesta.
Conjunction means that the sun is between the earth and Vesta.

Thanks, I looked it up before asking, but this line from Wikipedia made me think that it was the other way around so I didn't read the rest of the page:

two planets are in opposition to each other when their ecliptic longitudes differ by 180°

(copied from http://en.wikipedia.org/wiki/Opposition_(planets))

inertnet wrote:Thanks, I looked it up before asking, but this line from Wikipedia made me think that it was the other way around so I didn't read the rest of the page:

two planets are in opposition to each other when their ecliptic longitudes differ by 180°

Yes, it can be a little confusing, since the terms apply to a relationship between different bodies (usually referring to the position of two with respect to a third). If the exact bodies and their relationship aren't specified, the words by themselves can be ambiguous. In the Wikipedia example, two planets are in opposition to each other [with respect to the Sun] when their ecliptic longitudes differ by 180°. Vesta is in opposition to the Sun with respect to the Earth, so in that case the Earth and Vesta have the same ecliptic longitude.