Astro Bob: Space station bonanza — see both the ISS and China’s Tiangong – Duluth News Tribune

DULUTH — A bright light in the evening sky ushers in the new season this week. Spring begins at 10:33 am CDT, March 20, but before we get there, the space station will warm up the crowd. The football-stadium-sized orbiting laboratory will make one to two passes each night through the beginning of April. With warming temperatures and easy viewing times — most sky-crossings happen between 8 and 9:30 pm — everyone will have the opportunity to see and photograph the ISS.

Earth rotates from west to east, and since the space station travels in the same direction, it always rises somewhere along the western horizon, zips across the northern or southern sky and then “sets” in the east. Oftentimes, it disappears before officially setting because along the way it encounters the Earth’s shadow.

ISS eclipse

In this photo, the space station is moving from right to left. As it enters the Earth’s shadow, the sun sets for the spacecraft. The low-angled light turns the ISS orange and then red the same way a sunset colors the landscape on the ground.

Contributed / Bob King

Since we see the station by reflected sunlight, once it’s eclipsed by the shadow it fades and disappears from view. ISS eclipses are far more frequent than lunar eclipses because the space station is much closer to the Earth. From its viewpoint the shadow fills up a much larger part of the sky, so it runs into it all the time. At the moon’s distance of 239,000 miles (385,000 km), the shadow narrows down to just a few degrees across, so the moon (when full) must literally “hit the bulls-eye” for an eclipse to occur.

The brightness of the ISS varies depending on how far it is from the observer, and the angle it makes to that person. When passing directly overhead it’s closest at about 250 miles (400 km). But when it stands just one fist or 10° above the horizon, the ISS is 930 miles away (1,500 km)! The farther away it is, the fainter it appears.

Did you know the space station goes through phases similar to the moon? When it first “rises” in the west, the sun illuminates it from behind and a little off to the side like a crescent moon at dusk. When the ISS passes due south of north of us, it’s lit more like a half-moon. And when it’s standing off in the eastern sky, the sun shines fully upon it like the full moon. It shines brightest when it’s a little ways down in the eastern sky (nearly overhead) and in “waxing gibbous” phase.

ISS Duluth

This all-sky map from Heavens Above shows a bright ISS pass across the southern sky for Duluth, Minnesota starting about 8:54 pm local time on Thursday, March 17. The space station will pass directly over Sirius, the brightest star. Shortly after, it’s eclipsed by Earth’s shadow and disappears from view — the reason the path ends abruptly.

Contributed / courtesy of Chris Peat

To find out when the station will pass over your location, 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 two-dimensional representation of the sky dome, so its outer edge is the 360° horizon, and the center is the overhead point.

All times shown are local times on the 24-hour clock for your location, so 20:30 = 8:30 pm local time and 21:15 = 9:15 pm 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. Or sign up for alerts at NASA’s



Also, starting about March 17 and continuing through month’s end, China’s


(Heavenly Palace) space station will also be crossing the evening sky. At Heavens Above, instead of the ISS link, click on Tiangong. It orbits a little lower at around 236 miles (380 km) altitude, with its orbit inclined at 41.5°. That’s about 10° less than the ISS, making it not quite as widely visible around the planet.

Polar orbit ESA.jpg

Satellites in polar orbits usually travel around the Earth from north to south rather than from west to east, passing roughly over Earth’s poles. As the planet spins beneath, the satellite can study or map the planet’s entire surface.

Contributed / ESA, L. Boldt-Christmas

The steeper the orbit the more places the satellite is visible from the ground. Conversely, the more of the planet the satellite will see. That’s why spacecraft placed in steep polar orbits are excellent for surveillance and Earth observation. Each day, as the Earth rotates beneath them, their cameras and sensors can scan the entire surface.

Tiangong space station 2021 Shujianyang CC BY-SA 4.0.jpg

This rendering shows the Shenzou spacecraft (bottom) docked to the core module of China’s Tiangong space station. The Shenzou capsule ferried three “taikonauts” to the station on Oct. 15, 2021, for a six-month mission.

Contributed / Shujianyang, CC BY-SA 4.0

China launched the Tianhe (“Harmony of the Heavens”) core module last April, which can house three astronauts, called “taikonauts” or space travelers in Chinese. Several taikonaut crews have visited the station over the past year. In 2022, China plans to launch two additional laboratory modules to expand Tiangong so researchers can conduct experiments in a weightless environment.

Tiangong Duluth pass

Tiangong has a bright, magnitude 2.3 pass on Sunday, March 20 for Duluth, Minnesota from 8:37-41 pm local time. It also passes close to Sirius and then disappears in Earth’s shadow at 8:41 pm

Contributed / Chris Peat

Since it’s considerably smaller than the ISS, Tiangong isn’t as bright. During this month’s best passes it shines at second magnitude, the same brightness as the Big Dipper stars. And since the moon may be out during part of this week, it might be wise to keep a pair of binoculars handy.

From about March 17-26, both space stations will make passes each night at different times. Check Heavens Above for details for your location. Chinese and US politics may differ, but our goals in orbit align more closely.

“Astro” Bob King is a freelance writer for the Duluth News Tribune.

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