For several days next month, Venus will appear in the evening sky and and in the morning sky on the same day. I am having a great deal of difficulty visualizing how this is possible. It seems to me that Venus is either ‘earlier’ or ‘later’ than the Sun, but not both.
As with so many questions, I feel that I should be able to explain it, but can’t. Can you help lift the veil of ignorance from my brow?
Thanks.
I think Venus is at a certain point in its orbit where it rises and sets at about the same time as the sun because it’s close to the sun in the sky (meaning it looks close to the sun from our point of view). Since it’s visible for some time before sunset and some time after sunrise, we can see it both at dawn and dusk. It’s probably harder to see under these conditions because of glare from the sun than it is when it’s farther away from the sun in the sky.
I remember reading that the opposite is true: that the current angle between Venus and the sun is the greatest it’s been in X amount of time.
Where’s The Bad Astronomer when we need him??
Can you give us the dates, Karl?
We can check the times of the rise and set of the sun and Venushere.
I don’t follow Bob’s explanation at all. Like Karl, it seems to me that Venus eather has to rise before the sun, and be a morning star (in which case it also sets before); or set after, and be an evening star (in which case it rises after). I can’t visualize how it can be both.
AFAIK, March 24 to March 27.
Why does Venus have to rise and set before or after the sun? It doesn’t have to necessarily be “below” or “above” the sun. If it is as far up in the sky as the sun and either to the apparent left or apparent right of the sun, it should rise and set at the same time as the sun. In either case, it wouldn’t be visible for very long because of glare from the sun.
Looks to be that Venus and the Sun will be pretty close, so it is possible that the planet will be visible both in the evening and in the morning. Morning about a half hour before the sun rises, and it may be bright enough to be viewed about a half hour before sunset, but not after sunset. If I’m reading the Nautical Almanac correctly…
This would explain everything - Venus is still only on one side of the sun, but would be bright enough to be visible before sunset (and, before sunrise).
That was pretty much what I was trying to say, but I guess I didn’t phrase it very clearly.
The bad news:
your program seems not to be y2k compliant!
The good news is that the neato-keen JPL Horizons online ephemeris is:
Date__(UT)_HR:MN Azi(a-appr)_Elev a-mass APmag
2001-Mar-23 13:49 Cm 74.2491 -0.0249 31.792 -4.18
2001-Mar-23 13:50 Cm 74.3897 0.1753 29.867 -4.18
2001-Mar-23 13:51 Cm 74.5302 0.3756 28.091 -4.18
2001-Mar-23 13:52 Cm 74.6704 0.5760 26.454 -4.18
2001-Mar-23 13:53 Cm 74.8104 0.7766 24.946 -4.18
2001-Mar-23 13:54 *m 74.9503 0.9773 23.558 -4.18
And, of course, the proffered explanation is the correct one. It’s a neat trick, but in practice seeing something less than a degree above the horizon during a 4 minute window at dawn will be tough. Maybe on an east coast beach (though the times above are for LA)…
To further nitpick, Venus is easily doable during the daytime if you know where to look and have reasonable eyesight. Still a neat question, though.
I suggest you download a program called Skyglobe (planetarium shareware available from many places on the Internet). It can certainly help you visualize what’s going on. You will see that on the dates you mention, Venus appears to be several degrees above the plane of the ecliptic when viewed from the middle latitudes of the northern hemisphere. But when viewed from the middle latitudes of the southern hemisphere it appears to be several degrees below the plane of the ecliptic and may not be readily visible either as an evening or a morning star.
For a better understanding of why Venus can be both the morning and evening star, I suggest you check out the Solar System Live website. The first thing you should do is take a look at the positions of Mars, Earth, and the Sun between March 24th and 27th (you’ll have to enter the dates and click on update). Notice that all three bodies are roughly lined up during this time, and remember that in this standard top down view of the solar system:
1): Earth orbits the Sun counterclockwise.
2): Mars orbits the Sun clockwise.
3): Earth spins on its own axis counterclockwise.
Put all that together, draw a diagram to help yourself visualize the planetary movements, and you will realize how Venus can rise before the Sun in the morning and set after it at night. If you want to get a preview of how this phenomenon will actually look like from the ground, you can enter the respective dates and times into Your Sky and get a pretty good picture of the situation.
Big changes in the solar system since the last time I looked, I guess. I really should keep up with the latest.
Ooops 
Make all those Marses Venuses please 
So Venus orbits clockwise? When did that happen? I knew I should have renewed my subscription to Sky & Telescope…
Venus (and all major bodies) orbit the sun in a counter-clockwise direction (as viewed from the north). Venus rotates on its axis in retrograde (clockwise).
In Phobos link to Odenwald above, judging by Odenwald’s last paragraph, he is using the terms ‘evening’ and ‘morning’ stars the way I did in my OP. I thought that using that definition was the source of my confusion. (The problem goes away if replace ‘morning’ and ‘evening’ by ‘visible in morning’ and ‘visible in evening’.)