<p>As it happens, Wikipedia has an article on the moons of mars.</p>
<blockquote>
<p>If viewed from Mars's surface near its equator, a full Phobos would look about one-third as big as a full moon on Earth. It has an angular diameter of between 8' (rising) and 12' (overhead). Due to its close orbit, it would look smaller when the observer is further away from the Martian equator until it completely sinks below the horizon as the observer travels closer to the poles; thus Phobos is not visible from Mars's polar ice caps. Deimos would look more like a bright star or planet (only slightly bigger than how Venus looks from Earth) for an observer on Mars. It has an angular diameter of about 2'.</p>
</blockquote>
<p>The maximum angular resolution of a human eye is about half an arc minute.</p>
<p><a href="https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/" rel="noreferrer">https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/</a></p>
<p>So in some circumstances Deimos, appearing 2 arc minutes in diameter from the surface of mars, might appear to be sort of a solid object with a shape instead of a point of light like a star.  But it probably whould mostly look like a bright star.</p>
<p>So if someone saw Phobos, appearing a tiny object, pass in front of Deimos, it would be analogous to seeing the moon approach, occult, and then move away from a star.</p>
<p>And as it happens, the Wikipedia article on the moons of Mars has a real time video of Phobos approuaching Deimos, passing in front of it and then moving away from Deimos, taken by the Curosity rover in 2013. The whole video takes about 25 seconds.</p>
<p>The Curiosity rover is in Gale Crater, at 5.4 degrees south on Mars. And the two moons of Mars should orbit quite near to the equatorial plane of Mars.  So from there Phobos should often pass in front of Deimos.</p>
<p>Mars has an equitorial radius of about 3,396.2  kilometers or about 2,110.3 miles.</p>
<p>And the semi-major axis of the orbit of Phobos is about 9,376 kilometers.  So that means that Phobos is about 5,979.8 kilometers away when it is directly overhead on the surface of Mars, and it can been visible above the horizon when it is few thousand kilometers farther away.</p>
<p>Thus the maximum distance that real time astronomical events can be visible must be at least as far as the 5,979.8 kilometers which is the minimum distance to an eclipse or occultation of Deimos by Phobos as seeb from the surface of Mars.</p>
<p>Apparently it takes the Moon almost an hour to pass one lunar angular diameter along its orbit as seen from Earth.  That means that when the Moon occults a star there will be almost a hour between the star disappearing behind the Moon and the star reappearing from behind the Moon - if the full diamter of of the Moon passes in front of the star. And if the Moon merely grazes the star the occulation can take a much shorter time, possibly one that even the most impatient observers will consider to be &quot;real time&quot; events.</p>
<p>The Moon's orbit around the Earth has a perigree of 362,600 kilometers and an aphogee of about 405,400 kilometers.</p>
<p>So if Occulations of stars by the Moon are considered to be &quot;real time&quot; events the maximum distance at which astronomical events can be seen in in real time should be over 400,000 kilometers.</p>
<p>Of course the closest star which can be occulted or eclisped by the moon is the Sun.  The Earth's orbit around the Sun has a perihelion of 147,098,450 kilometers and an aphelion of 152,097,597 kilometers. Are there any purely solar events which can be seen in real time during a solar eclipse?</p>
<p>Any real time solar events should also be possible to film when observing the Sun through telescopes.</p>
<p>LIke ganules</p>
<p>&quot;In solar physics and observation, granules are convection cells in the Sun's photosphere. They are caused by currents of plasma in the Sun's convective zone, directly below the photosphere. The grainy appearance of the photosphere is produced by the tops of these convective cells; this pattern is referred to as granulation.&quot;</p>
<p>&quot;A typical granule has a diameter on the order of 1,500 kilometres (930 mi)<a href="https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/" rel="noreferrer">1</a> and lasts 8 to 20 minutes before dissipating.<a href="https://en.wikipedia.org/wiki/Solar_granule" rel="noreferrer">2</a> At any one time, the Sun's surface is covered by about 4 million granules. Below the photosphere is a layer of &quot;supergranules&quot; up to 30,000 kilometres (19,000 mi) in diameter with lifespans of up to 24 hours.&quot;</p>
<p><a href="https://en.wikipedia.org/wiki/Solar_granule" rel="noreferrer">https://en.wikipedia.org/wiki/Solar_granule</a></p>
<p>So a real time telescopic film of the Sun's surface would show granules appearing and dissappearing with a life span of 8 to 20 minutes. And possibly that might be fast enough to be considered to be real time for the purpose of this question.</p>
<p>The Sun usually has a few million spicules at any one time.</p>
<p>&quot;In solar physics, a spicule, also known as a fibril or mottle,[a] is a dynamic jet of plasma in the Sun's chromosphere about 300 km in diameter.<a href="https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/" rel="noreferrer">1</a> They move upwards with speeds between 15 and 110 km/s from the photosphere and last a few minutes each<a href="https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/" rel="noreferrer">1</a> before falling back to the solar atmosphere.<a href="https://en.wikipedia.org/wiki/Solar_granule" rel="noreferrer">2</a> They were discovered in 1877 by Angelo Secchi, but the physical mechanism that generates them is still hotly debated.&quot;</p>
<p>&quot;Spicules last for about 15 minutes;<a href="https://en.wikipedia.org/wiki/Solar_spicule" rel="noreferrer">3</a> at the solar limb they appear elongated (if seen on the disk, they are known as &quot;mottles&quot; or &quot;fibrils&quot;). They are usually associated with regions of high magnetic flux; their mass flux is about 100 times that of the solar wind. They rise at a rate of 20 km/s (or 72,000 km/h) and can reach several thousand kilometers in height before collapsing and fading away.&quot;</p>
<p><a href="https://en.wikipedia.org/wiki/Solar_spicule" rel="noreferrer">https://en.wikipedia.org/wiki/Solar_spicule</a></p>
<p>So maybe the rise and fall of a spicula in about 15 minutes is fast enough to count as real time.</p>
<p>Some of the most prominent solar phenomenon are prominenices and filaments.  I have seem films of prominences rising from the surface of the Sun, and then collapsing.  But I suspect those films were speeded up many times.  The shortest lasting prominences are active region ones.</p>
<blockquote>
<p>Active region prominences, or active region filaments, form within the relatively strong magnetic fields at the centers of active regions. Active region prominences have lifetimes ranging from hours to days and erupt more often than prominences belonging to the other classes.[9] As a consequence of being located within active regions, active region prominences are usually found in low heliographic latitudes.[10][11]</p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Solar_prominence#Classification" rel="noreferrer">https://en.wikipedia.org/wiki/Solar_prominence#Classification</a></p>
<p>Solar flares have different durations depending on which wavelengths are used to observe them.</p>
<blockquote>
<p>A common measure of flare duration is the full width at half maximum (FWHM) time of soft X-ray flux within the wavelength bands 0.05 to 0.4 and 0.1 to 0.8 nanometres (0.5 to 4 and 1 to 8 ångströms) measured by the GOES spacecraft in geosynchronous orbit. The FWHM time spans from when a flare's flux first reaches halfway between its maximum flux and the background flux and when it again reaches this value as the flare decays. Using this measure, the duration of a flare ranges from approximately tens of seconds to several hours with a median duration of approximately 6 and 11 minutes in the 0.05 to 0.4 and 0.1 to 0.8 nanometre bands, respectively.[15][16]</p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Solar_flare" rel="noreferrer">https://en.wikipedia.org/wiki/Solar_flare</a></p>
<p>So presumably a movie or video of a solar flare might show it lasting a short enough time to be considered a real time event.</p>
<p>Jupiter's orbit has a perihelion of about 740,595,000 kilometers or 4.5906 Astronomical Units (AU) and an aphelion of about 816,363,000 kilometers or 5.4570 AU, and thus the distance between Earth and Jupiter varies between roughly about 590,595,000 kilometers or 3.5906 AU and roughly about 966,363 kilometers or 6.4570 AU.</p>
<p>And many amateur astronomers enjoy watching the changing positions of the four large  or Galiean moons JUpiter.  As seen from Earth they seem to move back and fourth, passing alternately in front of Jupiter or behind Jupiter. The orbital periods range from 1.769 Earth days for Io to 16.689 Earth days for Callisto.</p>
<p>And if watched closely enough someone can see a moonappear to touch Jupiter or appear to detach from Jupiter.  They can seen moons appear to passe each other.  They can see the shadow of a moon appear on Jupiter or see a moon enter Jupiter's shadow and disappear.</p>
<p>It has always been much easier to determine the latitude of a place than the longitude.</p>
<blockquote>
<p>Galileo proposed determining longitude based on the timing of the orbits of the Galilean moons.[16] The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude. Galileo applied in 1616 for the Spanish prize of 6,000 gold ducats with a lifetime pension of 2,000 a year, and almost two decades later for the Dutch prize, but by then he was under house arrest for possible heresy.[17]: 15–16 
The main problem with the Jovian moon technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship, a problem that Galileo tried to solve with the invention of the celatone. Others suggested improvements, but without success.[15]</p>
<p>Land mapping surveys had the same problem determining longitude, though with less severe observational conditions. The method proved practical and was used by Giovanni Domenico Cassini and Jean Picard to re-map France.[18]</p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Galilean_moons#Determination_of_longitude" rel="noreferrer">https://en.wikipedia.org/wiki/Galilean_moons#Determination_of_longitude</a></p>
<p>Cassini &amp; Picard finished mapping France in 1684.</p>
<p>If the plane of the orbits of binary stars around their center of gravity happens to be lined up with the direction between them and Earth. each of the stars will pass in front of the other star (as seen from Earth).  So steller eclipses will happen twice in every orbital period.   Usually the light of both stars will be seen as a single point of light. And such eclipses will dim the combined light while one star is hidden behind the other.</p>
<p>Algol A &amp; B eclipse each other, causing their combined luminosity to vary, and the variablity was discovered in 1667, though their is speculation it was discovered much earlier.  British amateur astronomer John Goodricke discoverd the variabiity was periodic with a period of 2.86 days in 1783.  Algol is about 94 light years from Earth.</p>
<p>And today many amateur astronomers regularly observe the aparent brightness of variable stars and report them to professional astronomical organizations.  But I don't know if any variable stars have short enough periods to be be cosidere real time events.</p>

CC BY-SA 4.0
2024-07-22T13:12:05.280Z
heck of answer. i recall the book Longitude fondly, Harris' lonely struggle.
releseabe
CC BY-SA 4.0
2024-07-22T04:03:18.287Z
2024-07-22T04:03:18.287Z
M. A. Golding
<p>Note: I think I could make this question more interesting by asking, &quot;What is the phenomenon at greatest distance from Earth such that its movement can be perceived in real time?&quot;</p>
<p>My recollection is that every video I have seen shows distant phenomena as static. I guess this is because as fast as some things occur, they are very large things and the percent change is relatively small. I am not sure, but I think even comet tails which must be pretty active, would not appear to be moving to a human.</p>
<p>My guess would be that a real-time video of some solar events would appear to move. I do not know if the Jovian Great Red Spot looks static, but I think so.</p>
<p>I just read that the space telescope can see the Martian moons -- perhaps these would be perceivably moving?</p>

CC BY-SA 4.0
2024-07-21T04:13:44.063Z
Lunar occultation, especially a grazing occultation.
JohnHoltz
CC BY-SA 4.0
2024-07-21T06:35:05.640Z
Transit of Mercury or of Venus in front of the Sun (although the next one of Venus won’t be until the year 2117). The movement of Jupiter’s moons if you’re patient. Some variable stars, also if you’re patient, although that’s not “moving” per se, except if you count eclipsing variables, but then again you don’t *see* the *movement*, only its effect.
Pierre Paquette
CC BY-SA 4.0
2024-07-21T06:40:02.120Z
@PierrePaquette: If you were on the surface of Mars, do its moons move fast enough for a human to perceive their movement?
releseabe
CC BY-SA 4.0
2024-07-21T06:41:18.587Z
@releseabe: Yes (again, if you’re patient), but I was restricting myself to Earth-based astronomy.
Pierre Paquette
CC BY-SA 4.0
2024-07-22T15:51:59.217Z
A Solar eclipse is another one
Greg Miller
CC BY-SA 4.0
2024-07-22T16:13:28.493Z
@GregMiller yes, i think you see Luna's shadow subtly move. But imagine being able to perceive real-time  motion at, say, 4 light years distance. That may never be possible, I would guess.
releseabe
CC BY-SA 4.0
2024-07-23T00:26:10.620Z
I've added the [tag:time-domain-astronomy] tag. If you click it you'll probably find all kinds of goodies! [Gravitational wave events](https://astronomy.stackexchange.com/search?q=+%5Bgravitational-waves%5D+is%3Aquestion), [gravitational lensing events](https://astronomy.stackexchange.com/q/43234/7982), [Betelgeuse's dimming](https://astronomy.stackexchange.com/q/34547/7982). there's a [Nova predicted to become naked-eye visible this summer](https://astronomy.stackexchange.com/q/57236/7982), [visibly blinking neutron stars](https://astronomy.stackexchange.com/q/34561/7982)
uhoh
CC BY-SA 4.0
2024-07-23T00:36:02.267Z
(see also [Who first reported the Crab pulsar's pulsing but was dismissed by an astronomer?](https://hsm.stackexchange.com/q/11298/3869)) and don't forget the dance of Jupiter's Galilean moons that can watched in binoculars and was [used by Rømer to estimate the speed of light in 1676](https://en.wikipedia.org/wiki/Speed_of_light#Astronomical_measurementsr) We can watch sunspots evolve as they move across the sun (with a suitable projection system - NEVER look at the Sun through anything!)
uhoh
CC BY-SA 4.0
2024-07-23T00:42:26.900Z
Plenty of people [use their eyes to estimate brightness of variable stars as well](https://en.wikipedia.org/wiki/American_Association_of_Variable_Star_Observers) ([see also](https://astronomy.stackexchange.com/q/34482/7982)) Some stars [blink every 7 minutes](https://astronomy.stackexchange.com/q/43435/7982)!
uhoh
<jupiter><mars><comets><space-telescope><time-domain-astronomy>
Is there any astronomical phenomenon such that its movement can be perceived in real time?
releseabe
# Is there any astronomical phenomenon such that its movement can be perceived in real time?

(Asked by: username_0 on 2024-07-21)

Note: I think I could make this question more interesting by asking, "What is the phenomenon at greatest distance from Earth such that its movement can be perceived in real time?"

My recollection is that every video I have seen shows distant phenomena as static. I guess this is because as fast as some things occur, they are very large things and the percent change is relatively small. I am not sure, but I think even comet tails which must be pretty active, would not appear to be moving to a human.

My guess would be that a real-time video of some solar events would appear to move. I do not know if the Jovian Great Red Spot looks static, but I think so.

I just read that the space telescope can see the Martian moons -- perhaps these would be perceivably moving?

**Tags:** jupiter, mars, comets, space-telescope, time-domain-astronomy
> **Comments:**
> **username_1**: Lunar occultation, especially a grazing occultation.  
> **username_2**: Transit of Mercury or of Venus in front of the Sun (although the next one of Venus won’t be until the year 2117). The movement of Jupiter’s moons if you’re patient. Some variable stars, also if you’re patient, although that’s not “moving” per se, except if you count eclipsing variables, but then again you don’t *see* the *movement*, only its effect.  
> **username_0**: @PierrePaquette: If you were on the surface of Mars, do its moons move fast enough for a human to perceive their movement?  
> **username_2**: @releseabe: Yes (again, if you’re patient), but I was restricting myself to Earth-based astronomy.  
> **username_3**: A Solar eclipse is another one  
> **username_0**: @GregMiller yes, i think you see Luna's shadow subtly move. But imagine being able to perceive real-time  motion at, say, 4 light years distance. That may never be possible, I would guess.  
> **username_4**: I've added the [tag:time-domain-astronomy] tag. If you click it you'll probably find all kinds of goodies! [Gravitational wave events](https://astronomy.stackexchange.com/search?q=+%5Bgravitational-waves%5D+is%3Aquestion), [gravitational lensing events](https://astronomy.stackexchange.com/q/43234/7982), [Betelgeuse's dimming](https://astronomy.stackexchange.com/q/34547/7982). there's a [Nova predicted to become naked-eye visible this summer](https://astronomy.stackexchange.com/q/57236/7982), [visibly blinking neutron stars](https://astronomy.stackexchange.com/q/34561/7982)  
> **username_4**: (see also [Who first reported the Crab pulsar's pulsing but was dismissed by an astronomer?](https://hsm.stackexchange.com/q/11298/3869)) and don't forget the dance of Jupiter's Galilean moons that can watched in binoculars and was [used by Rømer to estimate the speed of light in 1676](https://en.wikipedia.org/wiki/Speed_of_light#Astronomical_measurementsr) We can watch sunspots evolve as they move across the sun (with a suitable projection system - NEVER look at the Sun through anything!)  
> **username_4**: Plenty of people [use their eyes to estimate brightness of variable stars as well](https://en.wikipedia.org/wiki/American_Association_of_Variable_Star_Observers) ([see also](https://astronomy.stackexchange.com/q/34482/7982)) Some stars [blink every 7 minutes](https://astronomy.stackexchange.com/q/43435/7982)!  

---

## 1 answers

### Answer by username_5 on 2024-07-22. Score: 6 | Accepted answer

As it happens, Wikipedia has an article on the moons of mars.

> If viewed from Mars's surface near its equator, a full Phobos would look about one-third as big as a full moon on Earth. It has an angular diameter of between 8' (rising) and 12' (overhead). Due to its close orbit, it would look smaller when the observer is further away from the Martian equator until it completely sinks below the horizon as the observer travels closer to the poles; thus Phobos is not visible from Mars's polar ice caps. Deimos would look more like a bright star or planet (only slightly bigger than how Venus looks from Earth) for an observer on Mars. It has an angular diameter of about 2'.

The maximum angular resolution of a human eye is about half an arc minute.

<https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/>

So in some circumstances Deimos, appearing 2 arc minutes in diameter from the surface of mars, might appear to be sort of a solid object with a shape instead of a point of light like a star. But it probably whould mostly look like a bright star.

So if someone saw Phobos, appearing a tiny object, pass in front of Deimos, it would be analogous to seeing the moon approach, occult, and then move away from a star.

And as it happens, the Wikipedia article on the moons of Mars has a real time video of Phobos approuaching Deimos, passing in front of it and then moving away from Deimos, taken by the Curosity rover in 2013. The whole video takes about 25 seconds.

The Curiosity rover is in Gale Crater, at 5.4 degrees south on Mars. And the two moons of Mars should orbit quite near to the equatorial plane of Mars. So from there Phobos should often pass in front of Deimos.

Mars has an equitorial radius of about 3,396.2 kilometers or about 2,110.3 miles.

And the semi-major axis of the orbit of Phobos is about 9,376 kilometers. So that means that Phobos is about 5,979.8 kilometers away when it is directly overhead on the surface of Mars, and it can been visible above the horizon when it is few thousand kilometers farther away.

Thus the maximum distance that real time astronomical events can be visible must be at least as far as the 5,979.8 kilometers which is the minimum distance to an eclipse or occultation of Deimos by Phobos as seeb from the surface of Mars.

Apparently it takes the Moon almost an hour to pass one lunar angular diameter along its orbit as seen from Earth. That means that when the Moon occults a star there will be almost a hour between the star disappearing behind the Moon and the star reappearing from behind the Moon - if the full diamter of of the Moon passes in front of the star. And if the Moon merely grazes the star the occulation can take a much shorter time, possibly one that even the most impatient observers will consider to be "real time" events.

The Moon's orbit around the Earth has a perigree of 362,600 kilometers and an aphogee of about 405,400 kilometers.

So if Occulations of stars by the Moon are considered to be "real time" events the maximum distance at which astronomical events can be seen in in real time should be over 400,000 kilometers.

Of course the closest star which can be occulted or eclisped by the moon is the Sun. The Earth's orbit around the Sun has a perihelion of 147,098,450 kilometers and an aphelion of 152,097,597 kilometers. Are there any purely solar events which can be seen in real time during a solar eclipse?

Any real time solar events should also be possible to film when observing the Sun through telescopes.

LIke ganules

"In solar physics and observation, granules are convection cells in the Sun's photosphere. They are caused by currents of plasma in the Sun's convective zone, directly below the photosphere. The grainy appearance of the photosphere is produced by the tops of these convective cells; this pattern is referred to as granulation."

"A typical granule has a diameter on the order of 1,500 kilometres (930 mi)[1](https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/) and lasts 8 to 20 minutes before dissipating.[2](https://en.wikipedia.org/wiki/Solar_granule) At any one time, the Sun's surface is covered by about 4 million granules. Below the photosphere is a layer of "supergranules" up to 30,000 kilometres (19,000 mi) in diameter with lifespans of up to 24 hours."

<https://en.wikipedia.org/wiki/Solar_granule>

So a real time telescopic film of the Sun's surface would show granules appearing and dissappearing with a life span of 8 to 20 minutes. And possibly that might be fast enough to be considered to be real time for the purpose of this question.

The Sun usually has a few million spicules at any one time.

"In solar physics, a spicule, also known as a fibril or mottle,[a] is a dynamic jet of plasma in the Sun's chromosphere about 300 km in diameter.[1](https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/) They move upwards with speeds between 15 and 110 km/s from the photosphere and last a few minutes each[1](https://neurologism.com/2020/01/24/is-the-resolution-of-the-human-eye-infinite/) before falling back to the solar atmosphere.[2](https://en.wikipedia.org/wiki/Solar_granule) They were discovered in 1877 by Angelo Secchi, but the physical mechanism that generates them is still hotly debated."

"Spicules last for about 15 minutes;[3](https://en.wikipedia.org/wiki/Solar_spicule) at the solar limb they appear elongated (if seen on the disk, they are known as "mottles" or "fibrils"). They are usually associated with regions of high magnetic flux; their mass flux is about 100 times that of the solar wind. They rise at a rate of 20 km/s (or 72,000 km/h) and can reach several thousand kilometers in height before collapsing and fading away."

<https://en.wikipedia.org/wiki/Solar_spicule>

So maybe the rise and fall of a spicula in about 15 minutes is fast enough to count as real time.

Some of the most prominent solar phenomenon are prominenices and filaments. I have seem films of prominences rising from the surface of the Sun, and then collapsing. But I suspect those films were speeded up many times. The shortest lasting prominences are active region ones.

> Active region prominences, or active region filaments, form within the relatively strong magnetic fields at the centers of active regions. Active region prominences have lifetimes ranging from hours to days and erupt more often than prominences belonging to the other classes.[9] As a consequence of being located within active regions, active region prominences are usually found in low heliographic latitudes.[10][11]

<https://en.wikipedia.org/wiki/Solar_prominence#Classification>

Solar flares have different durations depending on which wavelengths are used to observe them.

> A common measure of flare duration is the full width at half maximum (FWHM) time of soft X-ray flux within the wavelength bands 0.05 to 0.4 and 0.1 to 0.8 nanometres (0.5 to 4 and 1 to 8 ångströms) measured by the GOES spacecraft in geosynchronous orbit. The FWHM time spans from when a flare's flux first reaches halfway between its maximum flux and the background flux and when it again reaches this value as the flare decays. Using this measure, the duration of a flare ranges from approximately tens of seconds to several hours with a median duration of approximately 6 and 11 minutes in the 0.05 to 0.4 and 0.1 to 0.8 nanometre bands, respectively.[15][16]

<https://en.wikipedia.org/wiki/Solar_flare>

So presumably a movie or video of a solar flare might show it lasting a short enough time to be considered a real time event.

Jupiter's orbit has a perihelion of about 740,595,000 kilometers or 4.5906 Astronomical Units (AU) and an aphelion of about 816,363,000 kilometers or 5.4570 AU, and thus the distance between Earth and Jupiter varies between roughly about 590,595,000 kilometers or 3.5906 AU and roughly about 966,363 kilometers or 6.4570 AU.

And many amateur astronomers enjoy watching the changing positions of the four large or Galiean moons JUpiter. As seen from Earth they seem to move back and fourth, passing alternately in front of Jupiter or behind Jupiter. The orbital periods range from 1.769 Earth days for Io to 16.689 Earth days for Callisto.

And if watched closely enough someone can see a moonappear to touch Jupiter or appear to detach from Jupiter. They can seen moons appear to passe each other. They can see the shadow of a moon appear on Jupiter or see a moon enter Jupiter's shadow and disappear.

It has always been much easier to determine the latitude of a place than the longitude.

> Galileo proposed determining longitude based on the timing of the orbits of the Galilean moons.[16] The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude. Galileo applied in 1616 for the Spanish prize of 6,000 gold ducats with a lifetime pension of 2,000 a year, and almost two decades later for the Dutch prize, but by then he was under house arrest for possible heresy.[17]: 15–16 
> The main problem with the Jovian moon technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship, a problem that Galileo tried to solve with the invention of the celatone. Others suggested improvements, but without success.[15]
>
> Land mapping surveys had the same problem determining longitude, though with less severe observational conditions. The method proved practical and was used by Giovanni Domenico Cassini and Jean Picard to re-map France.[18]

<https://en.wikipedia.org/wiki/Galilean_moons#Determination_of_longitude>

Cassini & Picard finished mapping France in 1684.

If the plane of the orbits of binary stars around their center of gravity happens to be lined up with the direction between them and Earth. each of the stars will pass in front of the other star (as seen from Earth). So steller eclipses will happen twice in every orbital period. Usually the light of both stars will be seen as a single point of light. And such eclipses will dim the combined light while one star is hidden behind the other.

Algol A & B eclipse each other, causing their combined luminosity to vary, and the variablity was discovered in 1667, though their is speculation it was discovered much earlier. British amateur astronomer John Goodricke discoverd the variabiity was periodic with a period of 2.86 days in 1783. Algol is about 94 light years from Earth.

And today many amateur astronomers regularly observe the aparent brightness of variable stars and report them to professional astronomical organizations. But I don't know if any variable stars have short enough periods to be be cosidere real time events.

> **Comments:**
> **username_0**: heck of answer. i recall the book Longitude fondly, Harris' lonely struggle.  

---

