Your Sky Help: Ephemeris

The ephemeris shows you the positions of the principal bodies in the Solar System at the selected time, both in celestial coordinates, and as seen from your specified observing site for sky and horizon maps. If you're tracking an asteroid or comet by specifying its orbital elements, its position is given at the foot of the table. The sample ephemeris above gives positions for 00:00 UTC on March 1, 1995, while tracking the Earth-orbit-crossing asteroid 1995 DW1, whose discovery (on February 25, 1995) was reported in Minor Planet Electronic Circular 1995-E02 of 00:15 UTC, March 1, 1995, giving the orbital elements shown at the end of this page.

For each solar system body the following data are displayed: The first three items give the equatorial celestial coordinates of the object at the specified time, and its distance from the Earth. These values are independent of the observing site.

Right ascension

Hour angle at Greenwich.

Declination

Declination with regard to the equator.

Distance (AU)

Distance from the Earth in astronomical units (the Moon's distance is given in units of the Earth's radius).

The next three columns give the location of the object in the sky at the specified observing site. They are not included in displays from the Virtual Telescope, which uses celestial coordinates exclusively.

Altitude

Altitude of the body above the horizon, in degrees at the observing site. If negative, the body is below the horizon (“set”) and hence invisible.

Azimuth

Local compass bearing to the object at the observing site. Astronomers measure azimuth from the South (consistent with the convention for hour angle), with positive angles toward the West and negative angles toward the East.

Visibility

This column interprets the Altitude and Azimuth to describe what you can expect to see. Objects below the apparent horizon (taking account of the refraction of the atmosphere and the finite angular size of the Sun and Moon but not altitude of the observer) are shown as “Set” while objects clearly visible in the sky (or visible if the Sun weren't also up) are shown as “Up”. Objects that are just appearing or disappearing on the horizon are shown as “Rising” or “Setting”, and objects at culmination (crossing the local meridian) are shown as being in “Transit”. Transit times are quite accurate, but rising and setting times can be given only approximately because the altitude of the observer, local topography along the horizon, and the effects of atmospheric refraction (which, in turn, vary based upon temperature and barometric pressure) have a major influence upon when you will actually see an object appear or disappear at the horizon.

Press the button to reproduce the sample ephemeris above, using the following orbital elements for asteroid 1995 DW1:
1995 DW1 Epoch 1995 Feb. 12.0 TT = JDT 2449760.5 Marsden M 240.56097 (2000.0) P Q n 0.87220579 Peri. 325.86185 +0.71396325 +0.69743383 a 1.0849009 Node 349.21340 -0.54541801 +0.49845325 e 0.5139556 Incl. 19.34279 -0.43906227 +0.51490816 P 1.13 H 20.5 G 0.15