Not that it ever left us, but when the International Space Station appears at dawn, fewer people get to see it. But starting this week — and for many on this St. Patrick's Day — northern hemisphere sky watchers will see it with ease during evening twilight between about 7:30 and 9:30 p.m.
Depending on the station's exact altitude it takes between 90 and 93 minutes to circle the Earth, so astronauts working on board experience 16 sunrises and sunsets each day. Because the Earth rotates, the orbital track of the space station moves about 1,367 miles (2,200 km) to the west after each orbit.
For example, If you see it cross overhead from your house at say 7:30 p.m., during the next pass the ISS will lie 1365 miles farther to the west, equal to about 23° of longitude. By the time it arrives again at your place — 90 minutes later plus about five minutes extra travel time to cover that 1,365 miles — it will be following a different track in the sky.
The space station's orbit is tilted 51.6° relative to the Earth's equator, making it widely visible across both northern and southern hemispheres and accessible to some 90 percent of the planet's population. Satellites with lesser inclinations that orbit closer to the plane of the equator are only visible at equatorial and tropical latitudes.
Like all satellites, the space station is kept in orbit by Earth's gravitational pull. If the planet suddenly disappeared the station would fly off into space. Due to its greater distance from the Earth, gravity is about 90 percent of what it is on the ground.
The astronauts and everything else in the ISS are weightless because they're in free fall towards the Earth. The only reason the whole works doesn't crash is because the ISS is moving forward in its orbit at the tremendous speed of more than 17,000 mph (27,400 kph). Yes, it's falling but moving forward fast enough to avoid the plunge. If it suddenly stopped in orbit, the whole works would fall straight to the ground.
That said, the altitude of the ISS drops gradually over time due to the Earth's gravitational pull and a slight drag from the little bit of air still present at 250 miles up. To compensate, the astronauts have to periodically re-boost the craft to a higher orbit.
You can view the station now through about April 4. Some nights have one pass, others two. The ISS revolves around the planet from west to east, so it first appears in the western half of the sky and heads east, typically taking about 5 minutes for a full pass. Due to its size and reflective solar arrays it's normally about as bright as Jupiter and occasionally as luminous as Venus.
Here are some good passes for the northern Minnesota — Northwestern Wisconsin region in the next few nights:
- March 17 — Very low pass in the southern sky below Canis Major (the Big Dog constellation) and Sirius between 8:25-27 p.m.
- March 19 — Brilliant pass starting below Orion then crossing above Sirius all the way to Leo in the east between 8:27-31 p.m.
- March 20 — Another bright one! Climbs up the western sky then arcs high across the north starting at 9:17 p.m. Watch for it to quickly disappear from view at 9:19 p.m. when it's eclipsed by Earth's shadow. A pair of binoculars will show it change color from white to orange as the sun sets on the station.
How to find it from your home
To find out where and when to look from your home, go to Heavens Above and select your city by clicking on the blue Change your observing location and other settings link. Then return to the home page and click on the blue ISS link to see a 10-day table of passes that includes time, direction, brightness and altitude. Ten degrees (10°) of altitude is equal to one fist held at arm's length against the sky. The higher the negative number in the brightness column, the brighter the pass. Click on any pass time and a map will appear showing the station's path across the sky.
The map is a 2D version of the 3D sky. The outer edge is the horizon, and the center of the map is the overhead point.
All times shown are local times on the 24-hour clock for your location, so 18:30 = 6:30 p.m. and 19:15 = 7:15 p.m. You can also get a list of customized passes and alerts by downloading the free ISS Spotter app for iPhone and ISS Detector for Android devices.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.