Dancing the Lunar Transit

By Sarah Frazier
NASA’s Goddard Space Flight Center

On March 6, 2019, our Solar Dynamics Observatory, or SDO, witnessed a lunar transit — where both the Sun and Moon displayed a little odd behavior.

First, there was the transit itself. A lunar transit occurs when the Moon passes between SDO and the Sun, blocking the satellite’s view. But instead of appearing on one side of the frame and disappearing on the other, the Moon seemed to pause and double back partway through crossing the Sun. No, the Moon didn’t suddenly change directions in space: This is an optical illusion, a trick of perspective.

Here’s how it happened: SDO is in orbit around Earth. When the transit started, the satellite was moving crosswise between the Sun and Earth, nearly perpendicular to the line between them, faster than the Moon. But during the transit, SDO started the dusk phase of its orbit — when it’s traveling around towards the night side of Earth, moving almost directly away from the Sun — but no longer making any progress horizontally to the Sun. The Moon, however, continued to move perpendicular to the Sun and thus could “overtake” SDO. From SDO’s perspective, the Moon appeared to move in the opposite direction.

The second, subtler part of this celestial dance seemed to come from the Sun itself. If you look closely, you may notice the Sun seems to wiggle a bit, side-to-side and up and down, during the transit. That’s another result of SDO’s perspective, though in a different way.

SDO relies on solar limb sensors to keep its view steady and focused on the Sun. These limb sensors consist of four light sensors arranged in a square. To keep the Sun exactly centered in its telescopes, SDO is trained to move as needed to keep all four sensors measuring the same amount of light.

But when the Moon covers part of the Sun, the amount of light measured by some of the sensors drops. This makes SDO think it’s not pointed directly at the Sun, which would cause SDO to repoint — unless that function gets overridden.

Since SDO’s fine guidance system wouldn’t be much use during a lunar transit regardless, the mission team commands the spacecraft to disregard limb sensor data at the beginning of such transits. This loss of fine guidance accounts for some of the Sun’s apparent movement: SDO is now pointing at a general Sun-ward spot in space, instead of keeping its view steady using the much more accurate limb sensors.

The other factor behind the apparently wiggly Sun is temperature. SDO’s instruments are designed to work in the full glare of the Sun’s light and heat. When the Moon’s shadow passes over the spacecraft, the instruments quickly cool in the vacuum of space and start to bend and flex. The flexing of the front part of the telescope can make it look like the image is moving around in the frame.

SDO’s operators use strategically-placed heaters onboard the spacecraft to minimize this flexing as much as possible and to get back to providing science-quality data — images that are focused, centered and steady — as quickly as possible.

You can see and download SDO’s data — science-quality and otherwise — at sdo.gsfc.nasa.gov/data.