#1
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spring
I was plotting the sun on Cartes du Ciel around the time of the equinox this
morning and noticed the sun went past the 24hour RA at about 11:41 and declination did not not go above 0 until about 11:49. I expected them to be both at the same time as all info states it was at 11:44. is this a cartes du cirl error or am I mistaken in my beliefs and these do not coincide. thanks Paul |
#2
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spring
I do not know the validity of what Cartes du Ciel is showing you, but it is
true that the Sun does not necessarily pass through the equinox (a direction defined by the intersection of the plane of the ecliptic and the plane of the Earth's true equator). The Sun does not follow the path of the ecliptic exactly because of the way the latter is defined. My understanding is that the ecliptic is determined from the mean motion of the angular momentum vector of the Earth-Moon system relative to the Solar System barycentre; the ecliptic plane is then perpendicular to this vector. Some may qualify this ecliptic as "mean" to distinguish it from the "true" ecliptic, which is the instantaneous ecliptic, but in practice the former meaning is usually assumed. What distinguishes the mean ecliptic from the true ecliptic is the absence of short-term perturbations from other Solar System bodies. Notice how this definition does not involve the motion of the Sun as seen from the Earth, so it is not surprising to discover that the true path of the Sun will deviate from the ecliptic thus defined. What does the time of the equinox actually mean, then? For the vernal equinox it is the instant when the apparent geocentric ecliptic longitude of the Sun is zero, apparent meaning light-time delayed and aberrated position relative to the true-of-date equatorial frame. At this time the RA and DEC are not necessarily zero. Using DE421 planetary ephemeris and IAU-2006 earth-orientation model, I find the vernal equinox occurs at 11:43:37.6 UT1 (with a delta-T of 65.9 seconds). The ecliptic latitude at this time turns out to be -0.451 arcseconds, ie not zero! Also, the times at which the Sun reaches zero RA and zero DEC are 11:43:33.0 and 11:44:02.9, respectively, which do not agree well with the results you got from Cartes du Ciel. You will need to ascertain whether Cartes du Ciel is using the correct reference frame and centre of observation: the true-of-date equatorial frame and the geocentre, respectively. Maybe Cartes du Ciel is showing the results for a topocentric location and/or for the J2000 frame. In this case the position of the Sun could be even further displaced from the ecliptic. John. "Paul Forsdick" wrote in message ... I was plotting the sun on Cartes du Ciel around the time of the equinox this morning and noticed the sun went past the 24hour RA at about 11:41 and declination did not not go above 0 until about 11:49. I expected them to be both at the same time as all info states it was at 11:44. is this a cartes du cirl error or am I mistaken in my beliefs and these do not coincide. thanks Paul |
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