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Venus and Jupiter both appear to be of similar brightness in the sky, although, Venus appears to be slightly brighter than Jupiter. Sometimes when only one of them is in the sky, it is difficult to tell if it is Venus or Jupiter. Is there any way I could tell with certainty which planet is it? irrespective of the month of the year or location.

Obviously, I know about sky map related apps. However, spotting a planet with naked eye is more fun.

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    $\begingroup$ Once you have positively identified each of them, you will be able to recognize them only by their brightness. Venus gets much brighter than Jupiter, and Jupiter is much brighter than any star. Only Mars rivals with Jupiter sometimes. Nothing rivals with Venus. $\endgroup$ Commented Jun 30, 2023 at 0:23
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    $\begingroup$ For me, Venus is always unmistakably, strikingly white, while Jupiter is always unmistakably slightly yellowish - not quite white. However, that's a) not enough for an answer, b) not helpful at first, after a couple of decades of stargazing it becomes easier, and c) not necessarily true for anyone else. Brightness doesn't always work for me because sometimes on a dark night at opposition Jupiter also seems stunningly bright. Of course at those times, it's in the wrong place to be Venus. $\endgroup$
    – uhoh
    Commented Jun 30, 2023 at 9:35
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    $\begingroup$ @uhoh I mostly agree, except that all bodies look a bit red / orange when they're very close to the horizon. $\endgroup$
    – PM 2Ring
    Commented Jun 30, 2023 at 10:34
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    $\begingroup$ @PM2Ring ya - while we're keenly aware of that from the Sun during the day, it's easy to forget that it applies to the night sky as well. I guess that's folded into the experience or skycraft in caveat b) above. $\endgroup$
    – uhoh
    Commented Jun 30, 2023 at 12:07
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    $\begingroup$ @uhoh In fact I'd call Venus "blue-white" while Jupiter is much more muted and slightly reddish. In fact I'd say that distinguishing between Jupiter and Mars is more of a problem. $\endgroup$ Commented Jun 30, 2023 at 15:34

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This is "just" a matter of when and where.

A planet in the south at midnight - That's Jupiter.

A planet in the West shortly after sunset in the summer of 2023 - Venus.

There's no short cut here. You use your app or your almanac to determine when and where a planet will be and then you go to the sky and see it.

Now you'll get a sense, after a while, of the pattern in Jupiter's and Venus's motion. Jupiter moves slowly relative to the stars, so after a year and a month it is back to about the same position in the sky.

Venus moves from being an evening star to a morning star and then back again. It is never opposite the sun.

This might be called "skycraft". Learn your way around the sky and you'll come to learn when and where to expect to see a planet.

If you have a telescope, Jupiter is easily distinguished by its moons and its bands. Whereas Venus often has a clear crescent or gibbous shape, And, of course Venus is actually considerably brighter than Jupiter, so if they are side-by-side there would be no doubt which is which.

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    $\begingroup$ At the moment, it's not hard to see Venus an hour or so before sunset, if you know where to look. ;) $\endgroup$
    – PM 2Ring
    Commented Jun 30, 2023 at 6:41
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    $\begingroup$ "This might be called 'skycraft'" I see what you did there. :-) $\endgroup$
    – uhoh
    Commented Jun 30, 2023 at 9:38
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    $\begingroup$ "[Venus] is never opposite the Sun." It might be good to expand on this. I assume it's because the Earth never moves between Venus and the Sun, but I think it would improve your answer to explicitly say that (ie turning a tip into a logical reasoning). $\endgroup$
    – DukeSilver
    Commented Jun 30, 2023 at 18:39
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If the object is more than 47 degrees away from where the Sun is, then it must be Jupiter. If the object is within 47 degrees of the Sun then Venus would be much brighter than Jupiter and much brighter than any other star in the sky, whereas Jupiter would be only slightly brighter than the brightest few stars in the sky.

Explanation

Venus' orbit is interior to that of the Earth. From our point of view, Venus always appears reasonably close to the Sun in the sky and is either too close to the Sun to view (safely) or appears for a short time after sunset or a short time before dawn. The maximum angle between the Earth, Venus and the Sun is 47 degrees.

Jupiter's orbit is further out than Earth's. From our point of view Jupiter can be close to the Sun but also can be almost opposite to the Sun in the sky. Thus if the candidate object is more than 47 degrees from the Sun it must be Jupiter.

The brightness of both objects depends on the "phase" at which they are viewed, due to the illumination angle made by the Sun, and on the distance from Earth to the planet. Venus is much more variable in brightness than Jupiter because it can vary from being fully illuminated to a crescent and the relative variation in its distance from Earth is considerable (from 0.25 au to 1.74 au). The apparent magnitude varies between -4.92 to -2.98 at it's faintest (when it would be very close to the Sun and not really observable). Jupiter's distance from Earth varies from 3.93 au to 6.47 au and it is almost fully illuminated as observed from the Earth at all points in its orbit. The apparent magnitude of Jupiter varies from -2.94 at brightest to -1.66.

Thus Jupiter is always fainter than Venus, but it is only comparable with Venus at its faintest when it is in opposition to the Sun and hence closest to the Earth. But this means it couldn't be mistaken for Venus because it would be well over 47 degrees from the Sun. When it is closer to the Sun on the sky, its apparent magnitude will be about -2 and thus it will appear only a little brighter than the brightest stars in the sky (Sirius has an apparent magnitude of -1.5 for example). In contrast, Venus would usually be brighter than about apparent magnitude -3.5 when visible, and often brighter than -4, and would be obviously much brighter than any star.

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