HPWREN Cameras are ideal instruments for demonstrating additional astronomical facts
September 14, 2020
Robert Quimby, Director
Mount Laguna Observatory
Professor of Astronomy
San Diego State University
The HPWREN camera archive contains millions of images that, when formed into time-lapse movies, can be used to illustrate subtle changes over long periods of time. Many examples of this have been created with a focus on the Earth, for example a La Cima time lapse video of the vegetation regrowth over many years after the Cedar Fire in 2003, but the images also capture some of the sky above.
Previously the HPWREN images have been used to illustrate day-to-day changes in the position of the Sun (e.g. from sunset to sunset, or in 24-hour increments), but the upgraded HPWREN cameras at the Mount Laguna Observatory can now show the stars at night in stunning detail, making them ideal instruments for demonstrating additional astronomical facts. I have created four new videos showing how the stars appear to twirl overhead as the Earth spins, the apparent motions of the planets against the stars, and the passage of comet NEOWISE through the inner solar system.
The first video (above) begins with a time-lapse of the north-facing HPWREN camera at the Mount Laguna Observatory. The video shows the stars as they spin around the "North Celestial Pole", which is the point on the sky directly above the Earth's North Pole. The famous North Star (a.k.a. Polaris) can be seen near the top middle close to (but slightly offset from!) this point, and all the other stars appear to circle this point as the Earth spins. The second part of the video uses images from an all-sky camera that we have separately installed at the observatory. Making use of a fish-eye lens, this camera records the entire visible sky throughout the day and night. By aligning the images to the "Celestial Sphere" into which distant stars appear to be frozen, I try to highlight that it is the Earth that spins inside the Celestial Sphere and not the other way around as it may appear.
You may know that it takes the Earth "one day" to spin around, but the length of this day may surprise you. The second video uses HPWREN camera images to time the Earth's rotation against distant stars. Again using the observatory's all-sky camera the video illustrates the difference between a "solar day" and a "sidereal day". HPWREN images are used to find the length of a sidereal day, which is the time it takes for the Earth to spin once and bring distant stars back to the same position in the sky. Because the Earth orbits the Sun, we get one more sidereal day compared to solar days each year.
While distant stars return to the same place in the sky every sidereal day, some closer objects do not. In this third video the planets can be seen wandering across the Celestial Sphere from night to night. HPWREN images capture the slow, eastward motion of Jupiter and Saturn through the stars up to the point the Earth speeds past them in its tighter orbit about the Sun and the great planets appear to reverse their course. Closer Mars can also be seen flying through the HPWREN images. A version of this video set to music is also available.
Finally, the dramatic appearance of Comet NEOWISE was captured by the HPWREN cameras as well. This video strings together images from the north-facing HPWREN camera at the Mount Laguna Observatory to show how the comet moved through our sky from night to night. You will notice that the tail of the comet points away from the Sun since it is sunlight that gently pushes the dust and gasses released by the comet away from its nucleus.