by Axel » Fri Apr 23, 2010 10:28 pm
Dan Schroeder wrote:Axel wrote:Dan Schroeder wrote:
The multiple exposures in the photo were taken at intervals of one solar day, not one sidereal day. Retrograde motion is ordinarily defined with respect to the stars, not the sun.
Umm... That should not make any difference. The tables from which I get my information are based on Terrestrial Time (and I just rechecked on the IMCCE website).
It most certainly does make a difference. Again, retrograde motion is defined with respect to the stars (equatorial coordinates). If the stars were visible in this photo, the would be about a degree lower each day. So when Mercury is stationary with respect to the stars, it's already moving downward with respect to the sun.
Ah, right, now I see what you mean. I was confused by your reference to how retrograde motion is defined. In fact it is not defined with reference to the fixed stars; it is defined with reference to whatever coordinate system is in use for a particular case. So the turn-around in the picture is not technically a retrogradation, but a change in the distance between Mercury and the horizon at 19:50 UT. Thanks for pointing this out: it is very interesting. I am surprised that there is a week between the two phenomena - coordinate retrogradation and change of planet-horizon distance at sunset twilight.
FYI, from DE405 via IMCCE:
-- retrogradation in right ascension (apparent of date) sometime between 6:00 and 12:00 UT on 18 April 2010
-- change of direction in declination (apparent of date) sometime between 18:00 UT on 16 April and 0:00 UT on 17 April 2010
-- retrogradation in longitude (mean of date) sometime between 0:00 and 6:00 UT on 18 April 2010
[quote="Dan Schroeder"][quote="Axel"][quote="Dan Schroeder"]
The multiple exposures in the photo were taken at intervals of one solar day, not one sidereal day. Retrograde motion is ordinarily defined with respect to the stars, not the sun.[/quote]
Umm... That should not make any difference. The tables from which I get my information are based on Terrestrial Time (and I just rechecked on the IMCCE website).[/quote]
It most certainly does make a difference. Again, retrograde motion is defined with respect to the stars (equatorial coordinates). If the stars were visible in this photo, the would be about a degree lower each day. So when Mercury is stationary with respect to the stars, it's already moving downward with respect to the sun.[/quote]
Ah, right, now I see what you mean. I was confused by your reference to how retrograde motion is defined. In fact it is not defined with reference to the fixed stars; it is defined with reference to whatever coordinate system is in use for a particular case. So the turn-around in the picture is not technically a retrogradation, but a change in the distance between Mercury and the horizon at 19:50 UT. Thanks for pointing this out: it is very interesting. I am surprised that there is a week between the two phenomena - coordinate retrogradation and change of planet-horizon distance at sunset twilight.
FYI, from DE405 via IMCCE:
-- retrogradation in right ascension (apparent of date) sometime between 6:00 and 12:00 UT on 18 April 2010
-- change of direction in declination (apparent of date) sometime between 18:00 UT on 16 April and 0:00 UT on 17 April 2010
-- retrogradation in longitude (mean of date) sometime between 0:00 and 6:00 UT on 18 April 2010