If you see a bright ‘star’ slowly crawling across the sky in an arc from west to east, an object that doesn’t flash or possess red and green running lights like an aircraft, then you can be sure that you’ve spotted the International Space Station (ISS). The 400-tonne orbiting laboratory is so large — 73 × 109 x 20 metres — that it can be viewed with the unaided eye from the heart of the most light-polluted town or city. Fortunately, its current orbit carries it over the British Isles and Western Europe for the next few nights.
As an added bonus, the ISS passes close above an attractive conjunction of the 10-day-old Moon and Saturn late into the evening of Wednesday, 2 August 2017 for observers in the British Isles. On this night the ringed planet and waxing gibbous Moon lie just 5½ degrees apart, so the pair will fit in the same field of view of a low-magnification binocular. As seen from the heart of the UK, the International Space Station passes just 18 degrees — the span of an outstretched hand at arm’s length — above Saturn and the Moon a few seconds before 11:20pm BST.
If you have a clear quadrant of sky from the south to the west, look low in the western sky of the British Isles at 11:17pm for the spacecraft’s ascent — but be vigilant as the International Space Station is only in sunlight for a further 3½minutes before fading into the Earth’s shadow in the south close to 11:20pm + 30 sec. BST.
Using Astronomy Now’s Almanac to make ISS viewing predictions
Many of you may have used our online Almanac to obtain information about lunar phases, or the rising and setting of the Sun, Moon and planets for wherever you may live, but the Almanac can also tell you when and where to see the International Space Station.
In the 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 and — just as importantly — the Daylight Savings Time? box, if applicable to your time and location. The table underneath the month’s Moon phase data then shows current nighttime passes of the International Space Station over your chosen location during the next five days, if any.
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, weather permitting, you should see the ISS as a slowly moving, bright ‘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 the Earth’s shadow, respectively.
Here is an example from last year computed for the centre of the UK:In the example above, as seen from the heart of the British Isles on the evening of Tuesday, 2 August 2016, the ISS first appeared 16° (a span and a half of a fist at arm’s length) above the west-southwest (WSW) horizon at 10:09pm BST in a viewing window lasting five minutes. It attained a peak altitude of 50° above the south-southwest (SSW) horizon before sinking down to 15° above the eastern (E) horizon at 10:14pm BST. One orbit later, the ISS rose again at 11:46pm BST.
Note: the actual times of events in the future will change as the orbit of the ISS varies over time; Almanac predictions made on the day are more accurate.