Time travel is one of the best things about astronomy. Check out two websites that give skywatchers a more visceral sense of stellar distances and how constellations change shape across the sweep of time.

Sluggish sunshine
Every time we look at the Sun we see it as it was 8.3 light-minutes ago. 
Bob King

We all know the quickest way to travel back in time. Just look up at the stars. Wait. You don't even have to do that. Hold your hand 6 inches (15 cm) from your face and you see it as it was about a billionth of a second ago. We can't help but see into the past because light takes time to reach our eyes, even traveling at 300,000 kilometers a second.

That beautiful full Moon we saw over the weekend is 1.3 light seconds away, the Sun 8.3 minutes and the nearest star, Proxima Centauri, 4.24 light years. You wonder if there is such a thing as the present, when there's so much past.

Whenever I'm out with a group of students at night, I'll point to a bright star like Arcturus or Deneb and rattle off the year the starlight that streams into their eyes that night left its star. Since Arcturus lies 37 light-years away, when you catch its twinkle the next clear night, that light departed the orange giant in 1981. What I wouldn't give to have my 1981 hair back.

This interactive graphic depicts the stars of the zodiac constellations and the year the light left each of them. Hover over each star for more information.

Wouldn't it be nice if someone created constellation maps that would tell you how far back in time you were looking? Happily enough, an electronic components supply company called RS recently put up a page featuring the 12 traditional zodiac constellations with each star marked with the year its light left it. It's a unique and easy way to share and compare stellar distances at a glance. You might read that a star is 2,100 light-years away, but its distance becomes far more tangible if you know the light you see left in 100 BC, when Julius Caesar was born.

When using the site, the dates refer to the current year, 2018. For 2019 and beyond, add an additional year to the dates shown.

Seeing look-back dates bunched together like this makes for fascinating side-by-side comparisons. Consider the constellation Sagittarius, with Mu (μ) Sagittarii, the star just above the "Teapot," and Lambda (λ) Sagittarii, the star at the top of the pot. The ages of their respective light beams differ by more than 2,900 years!

Easy-peasy time travel
I looked up the distances of the Big Dipper stars to create a similar map. You can do the same.

Although the graphic is a fresh way to see the constellations, it depicts only a dozen of them. For others of your choosing, fire up a planetarium-style software program like Stellarium or Cartes du Ciel, zoom in on your favorite group, and screen-grab the image to your desktop. Then, use a web search engine to find the distance in light-years for each star in the image. Subtract their distance from 2018 to determine the year the light left the star. Add those dates next to each of the constellation's brightest stars using Photoshop or similar free options and bam, you've got a handy Powerpoint slide or facebook photo to share with fellow skywatchers.

As long as we're on the topic of time, there's another website you'll want to visit if you're heavy into time traveling like me. Stop by Tony Dunn's superb Proper Motion of the Constellations to see what the future holds for our favorite star patterns. Every single star in the sky we can see is a member of the Milky Way galaxy. Each revolves about the center of the galaxy with a particular speed and direction. Given enough time, all their individual motions will stretch and flex the outlines of the familiar constellations. In the far future, none will be recognizable. Although I'd love to live to see a millionth birthday, biology imposes certain limitations. That's why Dunn's site allows us a glimpse of eternity.

Proper motion exercise
It's fun-house mirror time! As we travel through time backwards and forwards, Sagittarius is transformed.
Courtesy of Tony Dunn with additions by author

Click on a constellation and watch its stars crawl to new places between about 30,000 BC to 30,000 AD. That's 60,000 years of cosmic travel. One click sets the clock running; a second click resets the clock. For an even more interactive experience, where you can select the constellation and clock speed, and travel even further forward and backward, visit Dunn's Orbit Simulator. Wait till you see how much of hurry Sirius and Procyon are in!

Fast-moving stars are often close to us. But not always. Vega's only 25 light-years away but appears to move very slowly because the Sun is headed in its direction. Stars that move across our line of sight appear to move much more swiftly compared to those that travel along our sight line.

Be sure to dress appropriately for your travels through the hills and valleys of spacetime. You can never go wrong with casual pants and comfortable shoes.


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May 2, 2018 at 11:49 am

If you click the Orbit Simulator link, here are instructions on how to use the simulator:

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Bob King

May 2, 2018 at 1:12 pm

Thanks for the extra link, Tony.

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May 3, 2018 at 10:18 am

Hello Bob King, my apologies for taking up your time but I need some help.
I am interested in visibility of stars in the vicinity of full moon. Do ynow if anybody has published obsrvations of the brightness of the sky around full moon?

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Bob King

May 3, 2018 at 10:43 am

Hi SkyT,
I don't know of any published observations, just anecdotal ones about how difficult it is to see stars the closer your gaze gets to the full moon. Since the closer you get, the fewer the stars I wonder if some application of the inverse square law might apply.

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Anthony Barreiro

May 3, 2018 at 3:21 pm

If I understand general relativity correctly, there is no absolute time dimension outside of four-dimensional spacetime. We see the Moon, Alpha Centauri, the Andromeda Galaxy, and quasar 3C 273 as they are right now in this moment in our frame of reference. If we were at the location of these objects it would be a second and a half earlier, or four years earlier, or 2.5 million years earlier, or 2.5 billion years earlier. But it's right now for us. And if we were travelling at the speed of light on one of those photons, It would be right now everywhere along the path from there to here and our clock would be standing still.

It's hard to wrap my mind around this perspective, because I move so much more slowly than a photon. But as cool as it is to think that by looking up at the sky we are looking back in time, it's also cool, maybe even more so, to think that I am perceiving the entire observable universe as it is right now and right here. Of course if the weather is cloudy, the universe looks smaller. 😉

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Bob King

May 3, 2018 at 9:19 pm

Hi Anthony,
Wonderful stuff to contemplate. My favorite line though is the last: "If the weather is cloudy, the universe looks smaller." My understanding is that if you were a photon, you'd be everywhere at once. Essentially, there's no distance. I'm a little unclear though on the 4D spacetime reference. Can you expand on your explanation?Those objects are still remote, and their light requires time to reach our eyes, so we do see them as they were long ago. Remote galaxies even appear different from those that are nearby.

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richard szweda

May 5, 2018 at 12:00 pm

Interesting that there seem to be no stars close to 2018 light years away

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Bob King

May 6, 2018 at 10:04 am

There are probably plenty, just not the ones shown in those particular constellation outlines.

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