On October 13, 2010, the Rossi X-ray Timing Explorer (RXTE
), a satellite in low-Earth orbit, observed the x-ray pulsar IGR J17480-2446 as it passed behind the Moon. This was an unusual opportunity to calculate the precise position of the pulsar by using the times at which it disappeared and reappeared at the edge of the Moon's disk.
As shown in this animation, ingress (the moment when the pulsar disappeared) occurred on the Moon's eastern limb just above the equator. Egress, 8 minutes 32 seconds later, was near the south pole on the western limb. The timing of ingress and egress depended delicately on the shape of the terrain. In other words, it mattered whether the pulsar passed behind a mountain or a valley. So the calculation relied on the detailed topography measured by both JAXA's Kaguya and NASA's Lunar Reconnaissance Orbiter (LRO
The animation faithfully reproduces the angle of the Sun, the position of RXTE, the position and orientation of the Moon as seen from the satellite, the Moon's topography, and the starry background. RXTE's position was derived from the Goddard Flight Dynamics Facility ephemeris for day 6129 of the satellite's orbit, while the Sun and Moon positions came from JPL's DE421 solar system ephemeris. All of the positions and the viewing direction were transformed into Moon body-fixed coordinates, so that in the animation software, the Moon remained stationary at the origin, while the camera moved and pointed appropriately. The Moon, the stars, the pulsar, and the clock were all rendered separately and layered together.
(A note about the name of the pulsar: The IGR prefix refers to INTEGRAL
, the satellite that discovered it. The numbers are its position in right ascension (hours, minutes) and declination (degrees, minutes). So the pulsar is at J2000 right ascension 17h 48m, declination -24°46'.)