New astrometric positions of the Jovian irregular satellites Elara and Pasiphae in 2016-2018

摘要

A total of 372 astrometric positions of Jovian irregular satellites Elara and Pasiphae were obtained during three successive oppositions in 2016, 2017, and 2018, by 1 m telescope at Yunnan Observatory. Based on Gaia DR2 catalogue, 196 new positions of Elara and 176 new positions of Pasiphae were collected. We compared our observations to the theoretical positions retrieved from IMCCE ephemeris online service with satellite ephemeris of Emelyanov model and planetary ephemeris DE431. For Elara, in each direction, the mean (O-C) is better than -0.004 arcsec and the dispersion is no more than 0.063 arcsec. For Pasiphae, in each direction, the mean (O-C) is better than -0.045 arcsec and the dispersion is no more than 0.082 arcsec. Our new observations have a smaller dispersion than previous observations. Moreover, we also compared two recent orbit models of each satellite as well as three different planetary ephemerides of Jupiter. These comparisons show that the differences among all the three planetary ephemerides are small.