Archive for June 2, 2022

06/02/2022 – Ephemeris – What’s an ephemeris?

June 2, 2022 Comments off

This is Ephemeris for Thursday, June 2nd. Today the Sun will be up for 15 hours and 22 minutes, setting at 9:22, and it will rise tomorrow at 5:59. The Moon, 3 days past new, will set at 12:34 tomorrow morning.

Yesterday, I was too busy on this program to mention that that program was the 47th anniversary of the Ephemeris program and was embarking on its 48th orbit of the Sun. At this juncture, you might be wondering: What’s an ephemeris? According to Wikipedia: Quote “In astronomy and celestial navigation, an ephemeris (plural: ephemerides; from Latin ephemeris, meaning ‘diary’, from the Greek, … meaning ‘diary, or journal’) gives the positions of… astronomical objects… at a given time or times. Historically, positions were given as printed tables of values, given at regular intervals of date and time.” enquote. My tables are now databases which I generate for the year during the prior December from published algorithms. I will show all on my blog today: (You are already here). I used to have to interpolate values from printed ephemerides for the first 5 or so years of the program.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). They may be different for your location.


An Ephemeris Example – Comet 73P-B/Schwassmann-Wachmann 3 nuclear fragment B

A sample of an ephemeris

Here’s an ephemeris for Comet Schwassmann-Wachmann 3 nuclear fragment B from the Jet Propulsion Laboratory’s (JPL) Horizons system. The listing has been truncated for width. Click on it to enlarge it.

R.A. is right ascension – East-west position in the sky, like longitude on the Earth, only it’s in hours, minutes and seconds.  One hour = 15 degrees.

DEC is declination – North-south position, in the sky, exactly like latitude on the Earth in degrees, minutes and seconds.

(a-apparent) means that the above coordinates are based on where the vernal equinox point in the sky is at that date and time, and for the observer’s location. Since I didn’t specify one, it’s the center of the Earth.

T-mag – Predicted total magnitude of the comet.  Magnitudes are like golf scores.  The higher magnitude, the dimmer the object.  It’s really, really dim.

N-mag – Predicted magnitude of the nucleus. No estimate is made here.

r – Distance from the Sun in terms of Astronomical Units (AU).  1 AU is Earth’s mean distance from the Sun.

rdot – The change in r.  It’s in kilometers per second.  If negative, it’s moving toward the Sun.

For more information on how I produce ephemerides for this program, go here: