If you chance upon a moving “star” rivalling planet Venus in brilliance, burning with a steady light that glides across the night sky from west to east, then you can be confident that you’re witnessing the International Space Station (ISS) – any object that flashes rapidly, or possesses red and green running lights is an aircraft. The ISS is easily seen from the most light-polluted city, and its current orbit enables it to be well seen from the British Isles and Western Europe over the next few nights.
ISS fact file:
The International Space Station orbits Earth every 92.7 minutes at an altitude that varies between 411 and 421 kilometres, travelling at an average speed of 27,500 kilometres per hour (7.6 kilometres per second). The spacecraft completes 15½ circuits of our planet each day in a path inclined to the equator by 51.6 degrees, meaning that it can appear overhead at all latitudes between 51.6° N and 51.6° S. For observers in the British Isles, the ISS can therefore be seen overhead at all locations south of a line drawn between Swansea and London, appearing lower in the sky for all places north of this line.Using Astronomy Now’s Almanac to make ISS viewing predictions
In our online Almanac, select the closest city to your location from the Country and City pull-down menus before ensuring that the box beside Add ISS passes? has a tick in it. The UK uses British Summer Time (BST) until 27 October in 2019, so the Daylight Savings Time? box should be checked. The table underneath the Moon phase data then shows any visible nighttime passes of the International Space Station over your chosen location during the next few days.
Here is a current example computed for Cardiff:For the given Date in year/month/day format, Local Time is the instant the ISS first becomes visible and Duration indicates the length of the sighting in minutes. At the given Local Time, look in the direction indicated by Approach and, assuming no clouds, you should see the ISS as a slow moving “star”.
Max. elevation is how high the Station will get above your horizon (90° is overhead, while 20° is about the span of an outstretched hand at arm’s length) and Departure indicates where the ISS will be when it vanishes from sight. Sometimes an appearance or disappearance occurs well up in the sky when the Station emerges into sunlight or slips into Earth’s shadow, respectively.
In the example above, as seen from the Welsh capital on the evening of Monday, 30 September, the ISS first appears 18° above the western (W) horizon at 8:22pm in a viewing window lasting three minutes. It attains a peak altitude of 67°, still in the west, before fading into Earth’s shadow 58° above the eastern (E) horizon around 8:25pm (all times BST).
Important note: the precise timing of an ISS pass is highly location-specific, plus the spacecraft’s track is subject to change due to periodic boosts to a higher orbit or to avoid debris, hence predictions made on the day are more accurate.