Proper motion is the apparent movement of stars with respect to one another as viewed from Earth. It takes decades, centuries or more for us to see a difference, because the distance moved in the sky is so small. If a star is moving directly toward or away from us, we would never see it move over time. Further discussion on the discovery of proper motion by Jacques Cassini can be found in Reference 1, and high proper motion Barnard's Star in References 2-4. One of the ways we can detect movement is to examine photographs taken over long periods of time. Let's look at an example in the constellation Orion.
In the evening of January 17, 2025 I pointed my 6-inch Celestron Origin Astrograph at the bright nebula NGC 1977, commonly known as the Running Man Nebula, in the constellation Orion. After 1 hour and 40 minutes integration time, a beautiful photograph was created and downloaded to my Samsung Galaxy S23 Ultra smartphone. It is easy to visualize this man running through the stars.
Date/Time: January 17, 2025, 10:18 p.m.
Location: Kempt Shore, Nova Scotia, 45°N
Sky: Partly cloudy, -6°C
Moon Phase: Waning gibbous (86%)
Technical: 10-second exposures, 1-hour & 40-minutes integration, line power, no filter, direct wifi
Three stars have been labelled in my image: Brun 910, 903 and 812. Brun 910 and 812 are high proper motion stars. This image was taken 130 years after the Riccò photograph was taken in 1895 (see below). The movement is small but easy to see.
In 1939, N.U. Mayall and H.W. Babcock took this photograph of NGC 1977 using the 91-cm telescope at the Lick Observatory. This was 44 years after the photograph by Riccò. Note that the three stars are starting to deviate from the straight line shown in the Hooker telescope's 1921 photo. This image, cropped from the full photograph of the Great Orion Nebula in Reference 10, was reproduced with permission. Photograph © UC Regents/Lick Observatory.
Photograph of NGC 1977 from the largest telescope in the world at the time, the 100-inch Hooker reflector at Mount Wilson Observatory. This date is 26 years after the Riccò photograph date. The original photographic plates are owned by Carnegie Science and the photo was taken on January 7, 1921. The exposure was 5 hours and 40 minutes. The three stars were essentially in a straight line. See Reference 9. Photograph used with permission. Courtesy of Carnegie Science, 13 March 2026.
In 1895, 130 years ago, A. Riccò published a photograph of this nebula in the Astrophysical Journal (Ref. 5). I have turned it upside down to make it easy to compare to my image. In this photograph, the three stars were almost in a straight line. © AAS. Reproduced with permission.
Additional images over 30 years old can be found in Refs. 6 & 7. An image more than 47 years old can be seen in Ref. 8. Unfortunately, dates were not provided for these photographs.