This app is an astronomical Wayback Machine for your smartphone

Contributed by
Sep 4, 2017

Want to blast yourself off back in time so you can see what the sky looked like when Galileo first saw the moons of Saturn through his telescope or Neil Armstrong’s boot first landed in a cloud of moon dust? There’s an app for that.

Astronomy apps like SkySafari 5, Star Walk 2 and Stellarium Mobile aren’t the TARDIS, but they can show you what the sky looked like during a specific date and time from a past era—if you know how to adjust them. These apps use the default location they get from your smartphone or other device. You can manually override the default settings on any of the better apps. SkySafari 5 has a Location option on its Settings menu, and Starwalk 2 has a current location option as well as a list of major world cities. Stellarium Mobile needs to have the Use GPS setting turned off before you can change other settings. You don’t need to be a perfectionist for this, either, because you only need to get within 60-100 miles of the location you’re aiming for.

You can manually adjust the settings of SkySafari 5 (left) or Stellarium Mobile (right).

Most of these types of apps allow you to change the date and time by manually entering them, though SkySafari 5 is the only one with a Now button that brings you back from time-traveling. Starwalk 2 has an infinitely cool scale that you can slide to take yourself into another era.

Say you want to look through the lens of the spyglass Galileo was peering through on that night in Padua on January 7, 1610. The stargazer observed Jupiter in a Paduan castle about an hour past sunset. That was when he spied three tiny cosmic objects floating next to Jupiter, two east and one west of the planet, which he thought were stars. The next night, they were all dancing to the west. What Galileo was actually seeing on that long-ago evening were the largest Jovian moons. Io, Europa, Ganymede and Callisto were sometimes elusive because he was unaware that moons could disappear while crossing a planet from the front or from behind. This also explains why he only glimpsed two or three moons in some of his observations. It was several nights later that he finally observed all four moons.

What Galileo’s observation of Jupiter on January 7, 1610 looks like from the SkySafari 5 app.

Go back to that moment by setting your app’s location to anywhere in Padua and the time to about 6 p.m. on January 7, 1610, then find and center Jupiter. Zoom in until you can see the planet and its moons. If you enable the display of the ecliptic line, you will be able to see it slopes to the west and that Jupiter lies below it because for such a huge planet it, has a small orbital inclination. As you jump from night to night until the 13th, you’ll see that Galileo missed some of moons because they were either in transit of their planet or each other.

You can use the same technique to recreate the momentous date of July 20, 1969, the first moonwalk. To see where Apollo 11 was on the moon, enable the display of surface features if your app allows it, and even see the Earth as Neil Armstrong saw it from the moon in SkySafari 5 by tapping the Orbit icon and you’ll find the display will give you the view from our satellite.

The first moon landing on SkySafari 5, with views of both what the moon looked like from Earth and vice versa. Zoom into the moon (inset) to see the Apollo 11 landing site.  

Want to know what the stars looked like on your birthday? How they could be aligned on Halloween? This could get really, really addictive.