Home > Concepts, Ephemeris Program > Ephemeris: 01/26/2024 – In astronomy “dark” means we can’t see it

Ephemeris: 01/26/2024 – In astronomy “dark” means we can’t see it

January 26, 2024

This is Ephemeris for Friday, January 26th. Today the Sun will be up for 9 hours and 33 minutes, setting at 5:42, and it will rise tomorrow at 8:07. The Moon, 1 day past full, will rise at 6:37 this evening.

What does the word “dark” mean? In astronomy the word dark means something we cannot see. In ages before 1959 we could not see the backside of the moon, so people got to calling it the dark side of the Moon. All changed in 1959 when a Soviet spacecraft went around behind the moon it took photographs of it. The two contenders for dark things in astronomy are now dark matter and dark energy. Dark matter is something that has a gravitational effect on the galaxies that it surrounds, but we can’t see it. Also, the expansion of the universe appears to be accelerating, where we would expect it to be decelerating because of gravitational forces of all the galaxies in it. We call that cause dark energy. And we don’t know what either is.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EST, UT –5 hours). They may be different for your location.

Addendum

Two possible causes of Dark Matter are in the running, WIMPs, and of course MACHOs. WIMPs are Weakly Interacting Massive Particles, while MACHOs are the bit more tortured acronym Massive Astrophysical Compact Halo Objects. Whatever they are they don’t interact much with themselves or ordinary matter, neither emitting light or any other radiation, or block it. However, they have mass and warp spacetime, distorting the shape of galaxies seen behind them. They also make galaxies seem to be more massive than their starlight would make them appear, and other effects.

Teaser Deep Field Image from President Biden's Presentation
The first deep field image from the James Webb Space Telescope shows a cluster of galaxies and a few stars. The stars have diffraction spikes, the rest of them are galaxies. The foreground galaxies of the cluster are white. The ones farther away are reddish, showing their red shift due to their greater recession speed with respect to the galaxy cluster we’re looking through. Note how those reddish galaxies are generally distorted into arcs whereas the nearer galaxies are not. This is the result of gravitational lensing caused by the gravitational fields of the galaxies and the dark matter between them. The warping of space by mass is a prediction of Einstein’s Theory of General Relativity, and one of its first proofs. Credit: NASA, ESA, CSA, JWST, STScI.
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