`Look! Up there in the sky! Is it a bird, is it a plane? No, it's Superman!" Well, maybe. But if something catches your eye like that nowadays, it is much more likely to be an artificial satellite, drifting silently across the night sky and glinting by the light of an invisible sun. Spacecraft are visible only when they are illuminated by the sun while at the same time the observer on the ground is in the dark. These conditions are met only when the sun is a little below the observer's horizon; if it is too far down, the satellite will also be in the Earth's shadow, and therefore not illuminated.
A consequence of this is that satellites, at this time of year, are visible only during a few hours after sunset, or in the corresponding period before sunrise. The most conspicuous man-made object in the sky at present is the International Space Station, or ISS. It circles the Earth about 240 miles above the surface, and takes an hour and a half or so to complete an orbit.
Most days it can be seen from Ireland; indeed if you live in Dublin and care to rise early from your bed tomorrow morning, and if there is no obscuring cloud, you will see it high in the sky, at an elevation of about 60 degrees, in the direction south-south-east at two minutes past five.
For other places the time of its appearance may be slightly different but can be found, as explained on this page yesterday, at the website www.heavens-above.com
Brighter and more spectacular than the ISS, albeit more ephemeral, are iridium flares. An iridium flare is a brief flash of celestial light caused by sunlight being reflected from the antenna of iridium spacecraft. There are more than 80 of these communications satellites in a variety of orbits, and their antennas are flat surfaces of highly polished aluminium; when all the angles are just right, the antennas reflect the sunlight like a mirror in the direction of an observer on the ground. Each "flare" lasts only an instant, but while there it is much brighter than any of the stars.
Again if you live in Dublin, you may wish to watch out for an iridium flare this evening. One is predicted for 19:58 p.m. from a satellite called Iridium 64. The flare will occur when the satellite is at an elevation of 60, with an azimuth of 152; in other words, look high in the sky as you face south-southeast. Full details of this too at www.heavens-above.com