Astroblog: Comets, Flat earth and Watching the Moon and planets move in a single night

Saturday, June 20, 2026

Comets, Flat earth and Watching the Moon and planets move in a single night

To be read in conjunction with the "Tangent" section of the astrophiz podcast #235
https://soundcloud.com/astrophiz/astrophiz-235-june-2026-sky-guide

There is a video going around on that wretched hive of villainy and scum called twitter labelled “Comets are not what we’ve been told”. It shows some very nice time lapses of bright comets with the caption ‘Watch how it moves in lockstep with the stars” …. And that’s it. That’s the argument.

Now, many people think because bright comets with the luminous tails look “swooshy” they must move perceptibly, like meteors. So perhaps the the lack of perceptible movement meant the comets were something else due to their naive expectations .... hence the earth is flat for .. reasons. But comets are exactly what we have been told (and the earth is not flat).

In a single night, when viewed from afar, most comets will not seem to move against the background stars, especially as the time window before they set is short. But through a telescope (or telescopic lens) you can see them move a bit. For C/2024 G3 and C/2025 R3 I had a roughly 20 minute window for imaging, so I wasn’t going to see much motion, but for C/20025 R3 I could compare my images with those by other astrophotographers in the Southern Hemisphere, and see for example, images taken a few hours previous in New Zealand showed the comet had move compared to my images.

And night after night they are against different stars as the follow their orbit. Comet C/2025 R3 was particularly lovely, not bright but drifting past the sword of Orion’s belt each night was memorable.


Comet C/2025R3 and Orion on the 10th May. Can't really see the tail against the light pollution. My image, long exposure (left) vs stellarium.	Comet C/2025R3 and Orion on the 10th May as simulated in stellarium.. Compare with my image, long exposure on the right.


Comet C/2025R3 and Orion on the 11th May. Sort of got a stubby tail against the light pollution. My image, long exposure (left) vs stellarium.	Comet C/2025R3 and Orion on the 11th May as simulated in stellarium.. Compare with my image, long exposure on the right.

Comet C/2025 R3, at the time it was crossing Orion’s belt, was moving around 1 degree every 24 hours, slow compared to the moon, which moves ~13.2 degrees every 24 hours, but not too different from the movement Venus, which moves on Average ~1.2 degrees per day and fleet Mercury which moves on average ~1.3833 degrees per day (0.5 degrees is roughly the diameter of the moon, which moves approximately one lunar diameter per hour). Mars moves ~ 0.5242 degrees per day and Jupiter, Saturn and the rest much less. This is why Jupiter and Saturn seem to stay in roughly the same place night after night. This is complicated by retrograde motion, where earth overtakes the planets in their orbits.

Mercury and Venus have a narrow window before they set to see them move in a single night. On the 20th and 21st you have about a 2 hour window to see Venus move against the background stars of the Beehive cluster, you do need a flat level horizon for this. (of course, day to day they move quite a bit with close approaches to other planets and stars over a matter of days).

Western sky on the evening of Saturday, June 20 as seen from Adelaide at 18:11 ACST (60 minutes after sunset, click to embiggen).

Venus is at its closest to the Beehive Cluster (M44).

Binocular view Venus and the Beehive at 18:11 ACST

Binocular view Venus and the Beehive at 19:41 ACST. Venus has slightly, but perceptibly moved against the stars of the Beehive.

We tend not to notice the relatively rapid movement on the Moon during the night, not may people are out looking at the sky for hours on end. Only when the moon is close to a very bright star do we sometime take notice.

On the 23rd you can follow the waxing Moon as it comes closer to the bright star Spica, in binoculars you will be able to see it come close to the star 49 Virginis over the span of a few hours. Of course on the 28th, in the early Morning you can watch the moon occult (or graze) Antares.


The Moon at the evening of Tuesday, June 21 as seen from Adelaide at 18:12 ACST (60 minutes after sunset, click to embiggen). The Moon is close to the bright star Spica	The Moon as seen through binoculars at 18:12 ACST (60 minutes after sunset, click to embiggen). The Moon is near the star 49 Virginis	The Moon as seen through binoculars at 22:00 ACST. The Moon is now beside the star 49 Virginis and visibly closer to Spica.