Retrograde motion

[JamesF]

I understand retrograde motion, but I'm not clear how it applies to what you see in the sky. For example, Saturn is currently in a retrograde phase I believe. Does that mean it appears to rise later each day instead of earlier (I think I have that the right way around , or can it also mean that it rises earlier, but not as much earlier as if it weren't in a retrograde phase?

I'm not even sure I've explained that very well. Hopefully someone will understand what I'm getting at.

James

[661-pete]

'Retrograding' applied to an outer planet, simply means that the Earth is 'overtaking' that planet in its orbit: because the Earth is travelling faster around its orbit than any of the outer planets, it does this every time the planet comes round to opposition.

When that happens, the planet appears to travel from east to west (i.e. left to right for northern observers) relative to the background stars, rather than the more usual west to east.

In the case of Saturn the retrograde period lasts about five months, but there is never a time when the planet rises later than it did on the preceding day.Because Saturn is such a slow mover, its times of rising and setting roughly follow sidereal time - that is, about four minutes earlier by solar time from one day to the next. This is pretty uniform throughout the year.

[kniclander]

good question nice to see an actual astro question rather than yet another my mount doesn't work..... humph humph.

and good answer

[661-pete]

and good answer

Thanks - but a far better and more comprehensive answer than mine, is only a Wikipedia-click away.

The phenomenon has considerable historical importance, in that it put Copernicus, followed by Kepler and Galileo, on the right track in realising the heliocentric model of the Solar System. Before Copernicus, the geocentrists were at their wit's end to explain the process, inventing 'epicycles' governing the planetary orbits. Even Copernicus didn't hit on the idea of elliptical orbits - that was down to Kepler. But at least he was able to explain retrograding.

[barkis]

For retrograde motion, Pete's explanation is correct.

Another interesting exercise, is to plot the Analemma.

Charting the suns position in declination throughout a year at specific intervals.

The result is a figure eight as depicted in the following APOD image.

Ron.

http://apod.nasa.gov/apod/ap020709.html

[182570]

good question nice to see an actual astro question rather than yet another my mount doesn't work..... humph humph.

and good answer

MY MOUNT DOESN'T WORK:hello2:

[661-pete]

Another interesting exercise, is to plot the Analemma.

Charting the suns position in declination throughout a year at specific intervals.

The result is a figure eight as depicted in nthe following APOD image.

Ron.

http://apod.nasa.gov/apod/ap020709.html

Ah yes - another, maybe little-known, phenomenon which is the most telling proof that the Earth's orbit is an ellipse (if it were a perfect circle, the two loops of the figure-eight would be the same size).

And somewhat neglected by amateur astros I fear! Is it just that few have the patience to set up a solar camera in exactly the same position, repeatedly throughout an entire year? Has anyone on SGL captured the analemma? I've done a quick search but can't find anything.

[barkis]

This might prove interesting to someone.

It was provided by MikeP in a thread on this subject a while ago.

Ron.

http://tinyurl.com/7wjyxbs

[kniclander]

MY MOUNT DOESN'T WORK:hello2:

the problem is probably to do with retrograde emotion

[Charon]

Excellent question James, which was well answered by Pete- well done to you both.

[LukeSkywatcher]

Retrograde motion is pretty confusing until you see/hear it explained so brilliantly that it makes perfect sense and you never forget how it works.

Antony A from Greece used to post a load of retrograde images on another website. I found them baffling and hadn't got a clue what they were until i asked him about them.

All made perfect sense.

I like to think of it as two cars on a motorway constantly overtaking each other.

[661-pete]

At the risk of opening up that old 'can of worms' once more, I notice from Wiki, that that 'other lot' consider retrograding to be a bad omen: if 'your' planet happens to be retrograding at your birth you're doomed or something. Oh dear! This is going to happen rather a lot. Saturn, Uranus and Neptune go retrograde for nearly half the time.