2020 Feb 18 Lunar occultation of Mars

For non-astronomers, “occultation” is a term that astronomers use for an eclipse of a star (or planet) by an asteroid or by the Moon.

The only lunar occultation of a major planet visible from North America at night during 2020 will occur Tuesday morning, February 18, when the Moon will occult Mars, but in eastern North America, the Caribbean, and northernmost South America, the event will occur after sunrise. Unfortunately, the server for the IOTA Web page for lunar occultations is currently down, so we are giving the predictions that were posted there, on this Web page. The server for the IOTA lunar occultations page should be back up in a few days, and then it can be consulted for predictions of lunar occultations of major planets and bright stars during the rest of 2020.

Region of visibiliy of the occultation of Mars, 2020 Feb. 18

The turquoise curves show where the disappearance or reappearance occurs at moonrise (left side) or moonset (right side); there is no northern limit as that misses the Earth to the north. The southern limit of the occultation, from which a partial occultation will be visible in a strip about 5 miles wide, crosses the n.e. Pacific Ocean and southern Mexico (white line, event at night), Central America (dark blue line that’s hardly visible, event during morning twilight), and northern S. America and the Atlantic Ocean (red dotted line, event during daylight).

View of the occultation from Kansas City

The occultation will be nearly central as seen from Kansas City in the above view of the 24% sunlit waning crescent Moon. The white dot on the left side shows where Mars will disappear [at 11:52:48 UT (Universal Time) = 5:52:48 am local time (CST)] on the Moon’s bright side; the sky will be reasonably dark since the Sun will be more than 12 deg. below the horizon (before nautical twilight begins). The white dot on the right side shows where Mars will reappear [at 13:20:43 UT = 7:20:43 am CST] shortly after sunrise (Sun altitude +2 deg.). The daylit sky will be quite bright so the dark side of the Moon will not be visible. From the Mountain and Pacific Time Zones, the reappearance will be a spectacular naked-eye event as Mars emerges from the dark side, faintly visible from Earthshine in the dark night sky.

For the eastern areas where the occultation will occur in daylight, the event might be seen with binoculars, but any small telescope will give a better view. Mars’ total magnitude will be +1.2 while its 5″ disk will be 91% illuminated. All of the predicted times of disappearance and reappearance given here are for the center of Mars. It will take about 15 seconds for Mars’ disk to be covered during the disappearance and uncovered during the reappearance, but these times will be slightly longer for locations well to the north or south where the occultation is not nearly central.

Tracks behind the Moon for 20 North American cities

As noted above, the disappearances are on the left (Moon’s bright side) and reappearances are on the right (Moon’s dark side). The numbers specify the cities as given in this list. For some locations (such as San Francisco, #17), the disappearance will occur before moonrise so its track on the left is not shown.

Lists of times and circumstances for hundreds of cities

The predicted times and circumstances for hundreds of locations are given in the above list. If you download it, the plain text file should be viewed with a fixed-space font such as Courier for the columns to line up properly. The times are Universal Time; subtract 4h for AST, 5h for EST, 6h for CST, 7h for MST, and 8h for PST. There are 3 lists, the first for the disappearance, the 2nd for the reappearance, and the last giving the coordinates (longitude, latitude, and height above sea level) for the cities. The first 2 letters given for the locations are the country code, so for US cities, go down to “US” and for Canadian cities, to “CA”. The city name is given (they are in alphabetical order by city name within the country), followed by the two-letter State code (only for US States).

For the disappearance and reappearance, the columns are described below:

  • U.T., h m s: Universal Time, hours, minutes, and seconds
  • Sun Alt: The Sun’s altitude in degrees, given only if it is higher than -13 deg. (that is, after nautical twilight begins). Of course, positive values means that it’s daytime.
  • Moon Alt: The altitude of the Moon above the horizon in degrees.
  • Moon Az: The azimuth of the Moon in degrees, the direction of the Moon measured around the horizon starting with 0 or 360 at due north; 90 is due east, 180 is due south, & 270 is due west.
  • CA o: Cusp Angle, deg. This is the angle of the event measured around the Moon’s disk from the nearest cusp, N indicating the northern cusp and S for the southern cusp. Negative values are on the Moon’s sunlit side (sunlit crescent) while positive values are on the dark side. If the Sun Alt. is -8 deg. or lower, the dark side should be faintly visible by Earthshine.
  • PA o: Position Angle, in degrees, the angle of the event measured around the Moon’s disk counter-clockwise from celestial north (towards the north celestial pole in the observer’s sky).
  • WA o: Watts Angle, in degrees. This is really Axis Angle, an angle measured around the Moon’s disk counterclockwise (like PA) but from the Moon’s northern rotation axis rather than from celestial north. Then very approximately, WA 0 is at the north cusp, WA 90 is approximately CA -90 (N or S), WA 180 is at the south cusp (CA 0S), and WA 270 is in the center of the dark limb (CA +90, N or S). For many years, we used Watts angle, which is the “axis” angle used by C. B. Watts in his epic 1964 charts of the Marginal Zone of the Moon, but soon found (from observations of lunar grazing occultations of stars) that it was offset from true axis angle by 0.2 deg. Since the charts were published (and were soon made available digitally), it was simpler to just offset axis angle, to take into account the error. But starting about 15 years ago, we began using the much more comprehensive lunar profile data that became available first from laser rangings from the Japanese Kaguya lunar orbiting spacecraft, and now in even greater detail from NASA’s Lunar Reconnaisance Orbiter. These data were all correctly referred to Axis Angle, and labelled as “AA” in most of our predictions, but for the current lists generated with IOTA’s Occult program, the column header was not changed from the old “WA”; Dave Herald, the author of the Occult program, will fix that in a later version; then we won’t have to give this explanation.
  • a m/o: “a” factor, the change of the event time with location longitude, in minutes per degree (or in seconds per arc minute) of longitude. The longitude is measured positive to the east of Greenwich, so all longitudes for this occultation are negative. The “a” factor, along with the “b” factor described next, can be used to calculate the time of the occultation for locations up to 50 miles from the place for which the times are given in the table.
  • b m/o: “b” factor, the change of the event time with location latitude, in minutes per degree (or in seconds per arc minute) of latitude.

You are encouraged to compute lunar and asteroidal occultation predictions (mainly of stars, quite frequent) for your location using IOTA’s free Occult software. UNFORTUNATELY, this link and the site from which the program can be downloaded, here, is the same one that is currently down due to server problems, IOTA’s lunar occultation Web site mentioned above, so for both, now you will just get an error. Hopefully, the server will be back up again in a few days and you can then obtain the software, and the information to use it. This .pdf document prepared by George Viscome is a good introductory guide for those wanting to get started with observing occultations, especially with the latest video equipment and techniques. As noted in it, the IOTA-VTI’s are available again, so we recommend them as sold through the IOTA store. Although observations (especially video or CCD) of lunar occultations of stars have some value, to assess and quantify any close duplicity, observations of lunar occultations of planets have no scientific value, except for the educational value of demonstrating the Moon’s orbital motion. Most of IOTA’s work now is with the prediction, observation, and analysis of occultations of stars by asteroids. For much information about this work, visit IOTA’s main Web site.

David Dunham, 2020 February 13 
e-mail: dunham@starpower.net
cell phone: 301-526-5590

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