Orwell Astronomical Society (Ipswich)
Geostationary Satellites, 08 October 2004 - 03 March 2025
A geostationary satellite travels in a circular orbit almost 36,000 km above the Earth's equator, following the direction of the Earth's rotation. It appears in a fixed position as seen from the surface of the Earth, and thus appears to move against the background stars. Geostationary satellites were first proposed by Arthur C Clark in 1945.
From the UK, the belt of geostationary satellites appears as a stationary ring with a declination of about -7.5°: anyone who has tried to photograph the Orion Nebula may have recorded horizontal streaks through their images, as the telescope follows the nebula and the stationary satellites appear to drift by. Geostationary satellites typically have a magnitude around 12.
Around 01 March and 12 October each year, the Sun is also at a declination of -7.5°. If a satellite sits "upright" in its orbit, shiny body panels and solar arrays can create a spectacular reflection of the Sun (although specific instances of reflection are not predictable). This is most pronounced just at the point where the satellite enters the Earth’s shadow (at these times of year, geostationary satellites are eclipsed by the Earth). When reflecting sunlight, the brightness of a satellite can increase considerably, up to magnitude 3, becoming potentially visible to the naked eye!
My observations of geostationary satellites are below.
Although a geostationary satellite nominally maintains a fixed position over the Earth’s equator, reality is not quite so straightforward. Perturbations due to the Moon and Sun slowly modify the orbit of the satellite, causing it to move around the nominal position.
On 25 February 2025, I decided to observe the motion of some geostationary satellites. I chose the Astra satellites that provide us with satellite television, locted at azimuth of 146.7°, altitude 25.4°. Although I didn’t need an equatorial mount to track their movements, using one enabled me to identify the background stars a little ahead of time then wait for the satellites to come into view before pausing the mount. Once I had located the satellites, I left the camera running until morning twilight.
The following video shows the stars rush by in the background as three Astra satellites slowly dance. Part way through, the Sun glints off the sides/solar panels of the satellites. There is a momentary blackout from clouds. As the latter part of the night arrives, the satellites become fainter as solar illumination becomes more side-on.
The following still shows tracks of the satellites throughout the night.
There are many Astra satellites and, on 03 March, I observed a group of five at azimuth 158°, altitude 28.3°. The following video shows the satellites slowly moving and occasionally flaring. There is a sudden loss of visibility (from 22:34 to 23:16) when the satellites enter the Earth’s shadow. As the night progresses the satellites become fainter as the solar illumination change from face-on to side-on.
The following image is the composite version. Each trail shows a break when the satellite is eclipsed.
The following video has a wider field of view. It shows the group of five satellites along with three other named and one unidentified satellite.
The following image is the composite version.
With the cameras staring in the same direction all night long, the level of background illumination in the images fortuitously provided an indication of light pollution over Ipswich. As the evening progressed, the background level dropped smoothly by a factor of two from 7.00pm until approximately 11.30pm, then was reasonably constant until dawn.
In the early evening of 01 March 2020, I took the opportunity to record the phenomenon using a Sony A7S camera with 100 mm lens on a driven but unguided mount. The entry point of satellites into the Earth’s shadow was at RA 10h 45m, declination -7.5°, among the faint star fields of Sextans.
In a two-hour period before clouds came in, I recorded 39 geostationary satellites, the vast majority in the equatorial belt with an inclination of zero. In the video below, the brightest star, λ Hydrae at magnitude 3.6, is to the bottom right. Several satellites appeared brighter, but the majority were in the range of magnitudes 5.5 to 7.0.
If satellites are the bane when photographing deep sky objects, then aircraft are the bane of wide field video photography. In the video, an aircraft would cross the field of view in six or seven frames. Approximately 6% of frames captured aircraft – they were edited out by hand.
The still photograph has a field of view sufficiently wide to capture the exit of satellites from the Earth’s shadow at RA 11h; this was an unintended bonus!
The following night, I observed again, using a longer lens, focal length 200 mm. The sky background varied throughout the three-hour observation so, rather than wrestle with that, I took a pair of frames and produced an absolute difference image. Ideally, this should contain two images of each satellite and no stars. However the stars are around the same size as the camera pixels and, depending how stars sit over the pixels, successive frames do not cancel out exactly.
I counted 66 satellites in the three-hour period, including the Astra satellites that provide satellite TV.
I took the following image on 08 October 2004 using a stationary (undriven) camera. It shows geostationary satellites spanning orbital slots from 16.0°E (Eutelsat W2) to 21.0°E (Afristar).
Nigel Evans