An ISS transit across the eclipsed Moon

by Michael Zeiler

On the morning of April 4th, a special opportunity exists for astrophotographers to capture the ISS transiting the Moon during the lunar eclipse. This will be a photogenic opportunity for persons with telescopes mounted with a video camera. 

In a message on March 29 to the Solar Eclipse Mailing List (SEML), John Irwin from the UK writes: “I spy another opportunity to catch the ISS transiting an eclipse, this time across the Moon during the lunar eclipse next Saturday, 4th April (UTC). There is no possibility of a transit while the Moon is totally eclipsed because the ISS is on the lit side of the Earth at that time (totality lasts only ~5 minutes anyway). But there's a favourable pass on the other side of the Earth between 51 minutes and 25 minutes before totality during which the Moon will be in deep partial eclipse.” These maps show the path of the transit using computations by John. Click on any map to see an enlargement. Scroll down to read more on how to read these maps.

More comments from John Irwin:

"I've updated the path coordinates for the ISS lunar transit based on new predicted orbital elements for Saturday. These new elements include the ISS orbit raising manoeuvre yesterday, the effect of which has pushed the transit path back to nearly its original position and time indicated in my previous set of predictions.

These are my final path predictions before the eclipse tomorrow. The data table can be found here: http:/www.jir1667.plus.com/shadow/iss-lunar-transit-central-path-2015-04-04.txt

Coordinates are given at 1-second intervals along the whole of the central path for which both the northern and southern limit exist. They are accompanied by the transit duration (seconds), path width (kilometres), azimuth and altitude of the Moon/ISS (degrees), and the angular diameter of the ISS (arcseconds).

In general I would recommend heading for the central line to maximise duration. If that is not possible, then try to keep to the south of the line to allow for your height above mean sea level. It would be a good idea also to keep well away from the path limits because the path may drift enough between now and the time of the transit to put you outside the path. Finally, it is anticipated that there will be some drift along the central path too, amounting to several seconds (possibly more), so you will need to take this into account when planning your observations.

As the transit path passes over a lot of high-altitude terrain, you may wish to make a correction to the position of the central path to allow for your local geodetic height. The approximate formula d=h*cot(A) can be used, where d is the path displacement, h is your height, and A is the altitude of the Moon, available from the path table. The direction of the displacement is given by the azimuth of the Moon, also given in the table. Note that this formula doesn't work well for very low lunar altitudes.

As an example, take the situation at Reno, Nevada where h = 1373 m. The transit here is predicted to take place at 11:28:22 UTC at which time A = 23.9 degrees. Then the central path will be displaced by d = 3.1 km at an azimuth of 239.3 degrees (reckoned from north through east). This is roughly 2 miles WSW."