Venus - Return to Evening Star?

Auld Jim Halpin

Hello All,

I very much miss the magnificent sight of Venus in the evening sky and in my research to find out when it finishes its morning shift and returns to evening skies I ended up more confused than enlightened. I read about the 548 day cycle from morning to evening star and back to evening star, approx 1.5 years, and also about direct and retrograde motion. Can anyone summarise Venus's pattern/motion in our skies without getting too technical? I'm not much of a morning person so viewing Venus in the AM is not really an option.

Regards,
AJH

mickmackey1

Well because Venus completes 8 orbits to every 5 orbits by Earth, there's a fair few mathematical gyrations involved. But you'll have to wait until November next year for Venus to once again become a brilliant evening star.

ps200306

Auld Jim Halpin wrote: »

Hello All,

I very much miss the magnificent sight of Venus in the evening sky and in my research to find out when it finishes its morning shift and returns to evening skies I ended up more confused than enlightened. I read about the 548 day cycle from morning to evening star and back to evening star, approx 1.5 years, and also about direct and retrograde motion. Can anyone summarise Venus's pattern/motion in our skies without getting too technical? I'm not much of a morning person so viewing Venus in the AM is not really an option.

Regards,
AJH

I'm the same. The only time I get to see Venus in the morning is when I've stayed up too late the night before. Hopefully without getting too technical, here's a picture of Earth's and Venus's orbits (stolen and modified from Wikipedia):



Suppose Venus is at inferior conjunction, i.e. on the same side of the Sun as the Earth, and in a line with them (as in the picture). If we are looking down from above the North Pole, then the planets will go anti-clockwise around the Sun, and the inner planets go faster. So Venus heads off to the right, immediately getting ahead of the Earth.

At some time in the future, Venus will overtake Earth on the inside, and the planets will be lined up once again with Venus at inferior conjunction. It won't necessarily be at the same place in their orbits of course (i.e. not vertically below the Sun in the picture, but somewhere else in their orbits). All we can say is that Earth will have done some (possibly fractional) number of orbits, and Venus will have done exactly one more orbit than the Earth in order to catch up with it.

So let Earth have done n orbits, and Venus n+1 orbits in the same period. Earth will take 365n days to do n orbits, and knowing that Venus's orbit take 225 days we can say:



A little bit of mucking around gives us:



That is the number of Earth orbits between inferior conjunctions of Venus, so we can just multiply by the Earth year length of 365 days to get 584 days between conjunctions. The same interval applies to successive occurrences of any part of Venus's orbit, for example "greatest western elongation" when Venus becomes our evening star.

To understand retrograde motion, you just have to remember that Venus is always overtaking us on the inside in our orbit. The retrograde motion of Venus refers to the apparent position of Venus in the sky against the (stationary) background stars. If you look at the line from Earth through Venus as Venus does its overtaking manoeuvre, you can see that it slopes one way then another with respect to the stars. So Venus will appear to move one direction, then stop and reverse, then continue as before:




You can see from the left half of the picture that the apparent size of Venus also changes, as you'd expect since it is moving closer and then further away. This gives a 3-D impression, and you can imagine the retrograde loop as the inner loop of a kind of Spirograph pattern (for those old enough to remember Spirograph :pac: ). That's exactly how it is if we put Earth at the centre of the universe and map Venus against the background stars we get the "pentagram of Venus":



The inner loops are the inferior conjunctions of Venus, and they repeat at the same position relative to the stars after every five occurrences.

The changing apparent size of Venus also brings up an interesting point about when Venus is at it's brightest. You can see from the top picture that Venus appears as a crescent when at its maximum elongation from the Sun. When it is closer to inferior conjunction Venus will have a bigger angular size (i.e. will appear bigger) but a smaller phase (i.e. less of a crescent). Trading off angular size against phase gives a complicated formula for the precise time of maximum brightness of Venus. However, the time of maximum elongation is when Venus will be in the darkest skies after sunset (or before sunrise) and is when Venus is at its most majestic, particularly if it occurs anywhere near an equinox (when the Sun "goes straight down").

As always, the best visualisation can be got by downloading the free Stellarium program. One of its neat features is being able to fix any object at the centre of the view. By fixing the Sun, and advancing one day at a time, you can see the orbit of Venus very clearly. By fixing Venus and advancing one day at a time, you can see its periodic retrograde motion very clearly.

coylemj

Auld Jim Halpin wrote: »

Can anyone summarise Venus's pattern/motion in our skies without getting too technical?

From 'Astronomical Phenomena' .......

2016:

VENUS is a brilliant object in the morning sky from the beginning of the year until the end of April after which it becomes too close to the Sun for observation. From mid-July it reappears in the evening sky where it stays until the end of the year. Venus is in conjunction with Mercury on July 16 and August 27,with Jupiter on August 27 and with Saturn on January 9 and October 30.

2017:

VENUS is a brilliant object in the evening sky until in the second half of March when it becomes too close to the Sun for observation. It reappears in late March as a morning star and can be seen in the morning sky until late November when it again becomes too close to the Sun for observation. Venus is in conjunction with Mars on October 5 and with Jupiter on November 13.

Auld Jim Halpin

Thanks Guys, that has explained a few things for me.

PS200306's post with the diagram explaining retrograde motion is an great visualization.

Coylemj, that is the sort of info I'm looking for, ill check out Astronomical Phenomena.

Cheers