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drhex
2009-Feb-02, 11:53 AM
When one watches the night-time sky with naked eyes in today's lightpolluted environment, only very bright objects can be seen. The brightest (apart from the moon) typically turns out to be Jupiter or Venus. I find out which one it is by running a program like Stellarium and giving it the long/lat of the current position and it already knows the time so it can generate a virtual sky.
Is there some rule of thumb that more experienced observers use to tell Jupiter from Venus without having to cheat with software or tables?

Tog
2009-Feb-02, 12:03 PM
Venus can only appear in the western sky after sunset or the eastern sky before sunrise. If it's bright and those times and positions don't match, it's probably Jupiter.

Venus is also a Brilliant white. Jupiter has a slight yellowish or tan tint to me.

Binoculars or any sort of magnification should reveal at least one moon of Jupiter as well.

George
2009-Feb-02, 03:46 PM
Venus can only appear in the western sky after sunset or the eastern sky before sunrise. If it's bright and those times and positions don't match, it's probably Jupiter. Yes. Venus can not get more than 47 degrees away from the Sun.

Fiery Phoenix
2009-Feb-02, 08:02 PM
In addition to what is mentioned above, Jupiter is in conjunction with the Sun at the moment -- which means it is no where to be seen from Earth. This leaves Venus to be the only remarkably bright object in the sky for a while; at least to anyone that lives in a light-polluted environment.

Cougar
2009-Feb-02, 08:23 PM
Pardon me if I tack a related question onto this thread. I've been watching Venus for the past few months. It's hard to miss. But over this time, certainly compared to Jupiter, Venus doesn't seem to be changing its position in the sky very much. Is its orbit and Earth's orbit kind of "in sync" temporarily? That is, on the same side of the Sun, orbiting in the same direction (of course), and near their closest approach to each other?

Tog
2009-Feb-02, 08:36 PM
As it reaches the point of greatest elongation (furthest angular distance from the sun) it can seem to "hang" for a few days or even weeks. Then it will start back towards the sun and the change from day to day can be very noticeable.

This is another difference between it and Jupiter. Jupiter's motion doesn't vary by anywhere near as much, but you do need to watch them both for a few days to really notice any movement that might be happening.

Hornblower
2009-Feb-02, 08:58 PM
Pardon me if I tack a related question onto this thread. I've been watching Venus for the past few months. It's hard to miss. But over this time, certainly compared to Jupiter, Venus doesn't seem to be changing its position in the sky very much. Is its orbit and Earth's orbit kind of "in sync" temporarily? That is, on the same side of the Sun, orbiting in the same direction (of course), and near their closest approach to each other?
They are no more "in sync" than at any other time, and still are far from closest approach. Venus is moving almost directly toward us for a few weeks, with little lateral motion. Late in March, around inferior conjunction, its apparent motion relative to the Sun will be almost entirely lateral, and will be at maximum angular velocity.

hhEb09'1
2009-Feb-02, 09:23 PM
Plus, Venus doesn't get much less bright than -4 magnitude, whereas Jupiter is a couple of magnitudes less bright, or even less. The only planet that gets near as bright as Venus is Mars, and it gets up to -3 rarely.

Swift
2009-Feb-02, 09:41 PM
Pardon me if I tack a related question onto this thread. I've been watching Venus for the past few months. It's hard to miss. But over this time, certainly compared to Jupiter, Venus doesn't seem to be changing its position in the sky very much. Is its orbit and Earth's orbit kind of "in sync" temporarily? That is, on the same side of the Sun, orbiting in the same direction (of course), and near their closest approach to each other?
This might help (http://www.fourmilab.ch/cgi-bin/Solar), it shows the current positions of the planets around the sun. You can see the relationship among Venus, the Earth, and the Sun.

m74z00219
2009-Feb-02, 10:12 PM
A corollary would be that for any object with a circular orbit and approaching 90deg elongation is either a superior planet (or what have you), a moon, or about to crash into your planet; in which case you had better hop onto your deluxe super fast spaceship.

Centaur
2009-Feb-04, 11:11 PM
Is there some rule of thumb that more experienced observers use to tell Jupiter from Venus without having to cheat with software or tables?

Venus is inside the Earth’s orbit and appears to oscillate on either side of the Sun over a synodic cycle of 1.6 years (time it takes to lap Earth) with an amplitude of about 46°. It alternates being seen in the west after sunset as an evening star and in the east before sunrise as a morning star. Near its greatest elongations its separation from the Sun does not appear to change much from day to day since its orbital movement is mainly along the tangent line connecting it with Earth. Its visual magnitude varies from around -3.9 near superior conjunction to about -4.6 during the period midway between inferior conjunction and either of its greatest elongation points. Greatest evening brilliance occurs around February 19th this year. Near some inferior conjunctions it briefly fades to a positive magnitude value.

Jupiter is outside of the Earth’s orbit and can appear in any direction relative to the Sun along the zodiac. It rises, transits and sets a few minutes earlier each evening. Jupiter’s visual magnitude only varies modestly from -2. That means it’s normally about seven or eight times dimmer than Venus. Nevertheless, it’s usualy the next brightest celestial object after the Sun, Moon and Venus except for rare instances for Mars.

Below is a graphic I created to illustrate the relationship among Venus, Earth and Sun during a synodic cycle.


http://www.curtrenz.com/VenusOrbit.JPG

JustAFriend
2009-Feb-05, 03:33 PM
Is there some rule of thumb that more experienced observers use to tell Jupiter from Venus without having to cheat with software or tables?

I dont consider it 'cheating' to use Stellarium or a copy of "Sky and Telescope"....

No one's grading you.

Cougar
2009-Feb-05, 04:56 PM
Venus is moving almost directly toward us for a few weeks, with little lateral motion. Late in March, around inferior conjunction, its apparent motion relative to the Sun will be almost entirely lateral, and will be at maximum angular velocity.

Aha. Thanks.

Cougar
2009-Feb-05, 05:00 PM
Below is a graphic I created to illustrate the relationship among Venus, Earth and Sun during a synodic cycle.

Wow. Excellent. Thanks.

mahesh
2009-Feb-05, 05:20 PM
You are a great mentor and a teacher, Mr Renz!
Thanks for sharing your time and efforts.

Centaur
2009-Feb-05, 09:18 PM
You’re welcome, Cougar and Mahesh.

Below is a chart I made to illustrate the mean elongation of Venus from the Sun relative to the number of days from inferior conjunction. “Mean” refers to averaged values that assume coplanar orbits with zero eccentricity. Reality during any particular apparition does not deviate much from the mean. It clearly demonstrates how little the elongation changes from day to day at the extremes.


http://www.curtrenz.com/VenusElong.JPG

m74z00219
2009-Feb-06, 12:18 AM
Hey Centaur! I'm just wondering, what software did you use to create that nice chart and graphic?

Centaur
2009-Feb-06, 04:35 AM
Hey Centaur! I'm just wondering, what software did you use to create that nice chart and graphic?

Glad you appreciate them, M74.

The software was self-written with the PowerBASIC programming language and associated Graphics Tools. For the latter I was a beta tester, and Perfect Sync incorporated my suggestion for a rotated ellipse function that allows me to draw Moon phases. The Astronomical Algorithms are principally derived from the book of the same name by Jean Meeus.