# Can you see something active in the sky apart from satellites? Can there be amateur time-domain astronomy?

What I mean is an event unfolding that is viewable by naked eye or telescopes, and doesn't take comparing days of footage to see moving pixels. Apart from satellites, is there something that moves or happens so quickly that humans could sense it within a reasonable time? The whole sky is like a still painting, I'd love to see parts of it being "animated". Things on my mind are:

• Crescent moon occulting a star (revealing or hiding it)
• Solar and Lunar eclipses

Or more exotic things like:

• Stars exploding (catching that as it happens, is it even possible?)
• Other planets' moons moving a reasonabl/detectable amount in 30 minutes perhaps
• Slow pulsar blinking (they are still super fast afaik)
• Star changing its brightness and not blinking due to haze/turbulence
• Exoplanet transit – Keith McClary May 17 at 5:12
• Of course you can see with the naked eye planets and stars rising or setting (and more generally, moving across the sky, but it may be less obvious) as the Earth rotates. – jcaron May 17 at 8:33
• With an ordinary DSLR with a decently long zoom lens, such as around 300mm or longer, you can see the moon move across the frame if you watch in live view. Not super spectacular but it's easy to see. – whatsisname May 18 at 5:55

If it moves or flashes it isn't astronomy, it is meteorology or technology.

There are only a few exceptions to this: Meteors are an atmospheric phenomenon, and a meteor will appear to move rapidly across the sky. But because they "come from space" and occur well above the clouds they are often considered to be part of astronomy.

As you note, eclipses and occultations happen quickly enough for the changes to be visible. The moons of Jupiter do move notably during the night, and when one moves into or out of shadow, it can appear or disappear over the course of a couple of minutes: easily noticeable.

A supernova could brighten quickly enough for the variation to be visible over a night's observation: You wouldn't see it suddenly appear, but over the course of a night it could appear brighter at the end of the night than at the beginning.

Similarly, Algol will fade from magnitude 2 to 3.5 over a few hours. It isn't quick enough to notice the change in the moment, but it is quite clear if you are observing over a few hours.

Pulsars are too dim to be seen with normal equipment even the bright nearby crab pulsar has a magnitude of 16.5 (and at one flash in 33 milliseconds, it is too quick to see by eye)

GRB 080319B was an exceptional object. It was a gamma-ray burst. It gave a flash of gamma rays that lasted a little over a minute. If you had happened to know exactly where to look you could, (marginally with the naked eye, but easily with binoculars) have seen the optical counterpart to the gamma-ray burst. This was created by the formation as a massive early star collapsed to a black hole producing a jet of energy that happened to point our way.

The sun is changing and on a small scale it does change on the scale of minutes, but it is hard to see any movement at that scale with basic equipment. If you can get your hands on a Hydrogen-alpha solar telescope you will, be able to see prominances that change over a period of hours.

Rarely an asteroid will pass so close so as to be visible. Apopsis will have a close approach in 2029 and will appear as a slowly moving star.

Jupiter is active at radio frequencies. You can tune in to Jupiter with the right equipment and hear it changing at rates from a few fractions of a second to a few seconds, see e.g. radiosky.com > Jupiter Central

These are exceptions. In general, it is rare for anything so big and powerful that it can be seen over a distance of many light-years to be able to change fast enough that those changes can be noticed by our eyes.

• Fainter GRB optical counterparts are accessible to amateurs skyandtelescope.org/astronomy-news/… – John Doty May 17 at 2:03
• On the Crab Pulsar in particular, it’s too quick to differentiate individual pulses by eye, but ~30 Hz is slow enough for some people to notice that it’s flickering. Of course, you still need a very big telescope (~550mm or larger aperture IIRC) to actually observe it, but there then are plenty of amateur astronomy societies that own observatories with big enough telescopes for that purpose. – Austin Hemmelgarn May 17 at 12:00
• Aren't we glad that GRB 080319B was billions of light years away. Aren't we, not only glad, but overall: We are, aren't we. – Peter - Reinstate Monica May 17 at 12:14

On September 20th, 2016, Victor Buso was testing his camera mounted on his 40-cm Newtonian telescope when he captured the first moments of a supernova. He was observing NGC 613, a spiral galaxy at a distance of 26.4 Mpc, because at that time it was located near the zenith. The exposure time was 20 s. An analysis of his images show remarkably fast rise rate in brightness of 43 ± 6 mag d$$^{−1}$$. This paper in Nature gives all the details. Bersten, M., Folatelli, G., García, F. et al. A surge of light at the birth of a supernova. Nature 554, 497–499 (2018). https://doi.org/10.1038/nature25151

https://doi.org/10.1038/d41586-018-02331-4

Many stars are double, and some orbit each other fast enough and far enough for amateurs to be able to detect them and measure them, and see the change in positions over the course of a few years.

On the scale of years to months to weeks to sometimes days, are also variable stars. James K did mention Algol in his answer, but there are literally thousands of variable stars. I suggest visiting the website of the American Association of Variable Star Observers—despite the name, it has members from around the world. https://aavso.org

If you have even a modest pair of binoculars and can hold them steady or support them against something so you can watch the four bright Galilean moons of Jupiter than you can watch them blink off and on again as the eclipse each other, i.e. pass through each others shadows!

This happens about twice every 12 years (Jupiter's orbital period) as the plane that contains Jupiter's satellites passes through Earth.

### And you are in luck for this!

These are happening RIGHT NOW! See answers to When will the next series of mutual eclipses of Jupiter's moons begin?

Actually the Moon occults bright planets at irregular but fairly frequent intervals. You can consult websites that predict occultations to find out more, but for fun see answers to Does a lunar occultation of Mars happen twice a year?

As @JamesK's answer points out you need a telescope to see pulsars blinking, but you also need to find a pulsar that blinks slowly enough that your eye can detect it. The Crab Nebula pulses at about 30 Hz and some people can notice it, but it would be better to choose one with a somewhat lower frequency. For more on that see answers to

### What about near Earth asteroid passes?

Answer(s) to Has a near earth object in heliocentric orbit ever been bright enough to be visible to the unaided eye? suggest we will be able to see some near Earth asteroids (NEOs) as they pass near Earth. In particular, from here:

99942 Apophis (2004 MN4) has an apparent magnitude between 1.7 and 2.7 based on these estimates. Its close approach date is April 13 2029.