My friend Jim sent me this photo of the International Space Station (ISS) when it crossed in front of the moon earlier this week. I’ve seen magnified photos of the ISS before but none that caught me completely off guard like this one. It looks as if the station is orbiting the moon, not the Earth! Looking closely, all eight large solar panels are easily visible as well as several modules that comprise the ISS and possibly the docked Soyuz spacecraft.
Many of us have thrilled to see the space station pass near a bright planet or star. But you can also know exactly when it will zip directly in front of the moon or sun in events called transits. To find out when the next one is happening over your home or within driving distance, use the ISS Transit Finder.
As you might guess, transits are infrequent because both the moon and sun are small targets in the sky. There are lots of near-misses. The transit finder will show you those, too. To get started, you’ll need to input your exact longitude and latitude. You can click either Auto Detect or Select from Map and that information will appear in the boxes (illustrated below) along with your observing site’s elevation.
Next, set the time span. Hover your cursor over the start and end date boxes and a black triangle will appear. Click the triangle for a calendar and then select a date. You’re only allowed a month’s forecast at a time, so if you set it for today (Feb. 9), you’ll see results through March 10. Then choose how far you’re willing to drive — if at all — and press Calculate. A list showing near-misses and (hopefully) transits will pop up, each replete with details that include:
- Transit time in 24-hour clock time.
- Distance to the ISS from the observer. The number includes both altitude and horizontal distance.
- Sun or moon’s elevation at the time
- Separation between the ISS and the moon/sun during a near miss or from the center of the moon/sun during a transit.
- Azimuth (compass direction) and altitude of the moon/sun at the time
- Distance from the center line, where the ISS would pass through the center of the moon/sun
- Width of the path where you can see the transit
A handy tip: to return to the settings page, click the Return to Settings button, not the back button.
ISS moon transit on Jan. 12, 2020 at normal speed and slowed down
The ISS is only about one-half of one arc minute across (a full moon is 30 arc minutes wide), and a typical transit lasts only about a second. The best way to watch one is with a pair of 10x binoculars on a tripod or a small telescope. For solar transits, be sure to place dual solar filters over the big objective lenses on the binoculars or over the front end of the telescope to protect your eyes from damage. Although the ISS is small and moves swiftly, it’s also extremely bright, so you can easily follow it with your eyes and anticipate exactly when it will cross the moon/sun.
When the ISS passes in front of a bright, nearly full moon or the sun it appears in black silhouette. Binoculars will show it as a dark speck; in a telescope you’ll get a quick glimpse of its shape. But if you’re lucky and it transits the lunar crescent you’ll see it fully illuminated against the dim, earth-lit portion of the moon even in handheld binoculars like the photo above.
Method 2: CalSky
You can also get a list of transits and other details at CalSky Select your location, then under the headings at the top of the page, choose Satellites and then the Internat. Space Station ISS link. Choose a duration of two weeks and press Go. Scroll through the list and look for any moon and sun crossings. If there is one, click on the Centerline link for a map of the centerline and other details.
Good luck and let us know if you have success!
Starlinks … again!
As I wrote in an earlier blog, the fourth Starlink volley, launched on Jan. 29, is currently visible in the morning sky around the start of dawn from many U.S. locations. To find out when and where to see the best passes go to Heavens Above and select your city by clicking the Change your observing location and other settings on the left side of the page. Then return to the landing page and click the Starlink – all objects from 4th launch link.
Scroll through the lengthy list and select the brightest passes when the satellites will shine around magnitude 2.2. Those will be the easiest to see. The times are shown, and if you click anywhere on the satellite entry, a map will pop up showing when and where to look. Because the moon is at or near full the next few nights, it will brighten the sky, but magnitude 2 satellites will still be obvious.