See the International Space Station from the UK

Like any object in low Earth orbit, the International Space Station (ISS) can can occasionally pass in front of (transit) more distant objects — in this case, the Moon. The ISS (circled) is seen in silhouette as it transits the lunar surface at roughly five miles per second at 4:41am EDT on Sunday, 2 August 2015 as seen from Woodford, Virginia, USA. Onboard the ISS are; NASA astronauts Scott Kelly and Kjell Lindgren: Russian Cosmonauts Gennady Padalka, Mikhail Kornienko, Oleg Kononenko, and Japanese astronaut Kimiya Yui. The International Space Station makes several visible passes over the British Isles during the first ten days of August. Image credit: NASA/Bill Ingalls.

While most celestial objects require a telescope or binocular to see clearly, there is one bright, man-made object that makes regular visits to the night sky that can be enjoyed with nothing more than the naked eye, whether you live in the country or the heart of a light-polluted city — the International Space Station, or ISS. It so happens that the ISS will be well seen from the British Isles during the first part of August, appearing like a slowly moving bright star, crossing the sky in an arc from west to east.

An engineering and scientific marvel of our age, construction of the ISS began with the launch of the first module, Zarya, in November 1998. Since then, more than 115 constructional space flights using five different types of launch vehicles have led to the 73 x 109 x 20 metre, 420-tonne structure that we see today. The orbital outpost has been continuously occupied since November 2000 and has seen more than 200 visitors from 15 countries.

The International Space Station on 23 May 2010 as seen from the departing Space Shuttle Atlantis during STS-132. Image credit: NASA/Crew of STS-132/Wikimedia Commons. — The International Space Station on 23 May 2010 as seen from the departing Space Shuttle *Atlantis* during STS-132. Image credit: NASA/Crew of STS-132/Wikimedia Commons.

The International Space Station orbits the Earth every 92 minutes at an altitude between 205 and 255 miles, travelling at an average speed in excess of 17,000 miles per hour (7.5 kilometres per second). The ISS orbits our planet with an inclination to the equator of 51.6 degrees, meaning that it can appear overhead at all latitudes between 51.6° N and 51.6° S. As far as the UK is concerned, the ISS can be seen as high as the zenith 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
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.

Here is an example computed for Edinburgh: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 Earth’s shadow, respectively.

In the example above, as seen from the Scottish capital on the evening of 2 August, the ISS first appears 14 degrees (a span and a half of a fist at arm’s length) above the southwest (SW) horizon at 11:17pm BST in a viewing window lasting four minutes. It attains a peak altitude of 30 degrees above the southern (S) horizon before fading into the Earth’s shadow 21 degrees above the east-southeast (ESE) horizon at 11:21pm BST.

Note: the actual times of events in the future will change as the true orbit of the ISS varies over time; predictions made on the day are more accurate.

If you need assistance visualising how the ISS moves through the sky, perhaps this fictitious example will help:

In this representation of an ISS track projected onto the sky as seen from a Northern Hemisphere location, the International Space Station first appears ten degrees above the west-southwest (WSW) horizon, reaches a peak altitude of 66 degrees above the northwest (NW), before disappearing into the Earth's shadow 31 degrees above the northeast (NE). Image credit: NASA. — In this representation of an ISS track projected onto the sky as seen from a Northern Hemisphere location, the International Space Station first appears ten degrees above the west-southwest (WSW) horizon, reaches a peak altitude of 66 degrees above the northwest (NW), before disappearing into the Earth’s shadow 31 degrees above the northeast (NE). Image credit: NASA.

Viewing the ISS through a telescope?
Given the International Space Station’s physical size, proximity, and brightness (at its best it can rival Venus), one might suppose that it would be fine object to see through a telescope. Indeed, when passing overhead at a favourable orientation, it can exceed an arcminute in angular size — about the same as Venus close to inferior conjunction.

The International Space Station photographed through a telescope by astrophotographer Ralf Vandebergh of the Netherlands. "The image shows not only the ISS with very special lighting angle but also it shows activity around the ISS which is often the case. You see the Japanese Cargo Ship HTV-1 in its demonstration flight shortly before docking and just a few hundred metres below the ISS," he said. Image credit: Ralf Vandebergh / Wikimedia Commons CC BY-SA 3.0. — The International Space Station photographed through a telescope by astrophotographer Ralf Vandebergh of the Netherlands. “The image shows not only the ISS with very special lighting angle but also it shows activity around the ISS which is often the case. You see the Japanese Cargo Ship HTV-1 in its demonstration flight shortly before docking and just a few hundred metres below the ISS,” he said. Image credit: Ralf Vandebergh / Wikimedia Commons CC BY-SA 3.0.

In practice, trying to track the ISS with an undriven telescope such as a Dobsonian feels rather like clay pigeon shooting, as one attempts to anticipate where it will be in the finder before catching a glimpse as it flashes through the field a high-power eyepiece in a fraction of a second. However, practice makes perfect, and if a carefully focused astrovideo camera is used in place of an eyepiece, some frames on playback can contain tantalising images.

Observers with computerised mounts that can be driven by external software designed to track satellites a have much greater chance of success. Astrophotographers Thierry Legault and Ralf Vandebergh have succeeded in capturing stunning pictures of the ISS showing fine structural detail that are an inspiration to imagers the world over.

As I said at the beginning of the piece, the beauty of the International Space Station sailing silently overhead needs nothing more than the naked eye to appreciate. Furthermore, if you do make a sighting, contemplate the six-person crew of Expedition 44 that are currently aboard. Clear skies!