h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A few things I need to point out there:

1. The moon goes around the Earth every 28 days rather than 29.5, if it is position relative to the stars you're talking about. So if the moon is beside Jupiter, it will be back in that position 28 days later (Jupiter is slow-moving and won't have moved much in that time).

The 29.5 comes from the fact that the earth goes around the sun, so the sun will be in a different position after 28 days, so the moon has to go for another 1.5 days before it has caught up.

2. You talk about the Earth and Venus being on the same side of the ecliptic. Don't know what you mean there. The ecliptic is the imaginary line in the Earth's sky that the sun travels along, which is a circle. The Earth is not on either side of the ecliptic, it is at the centre of it.

"Ecliptic" can also refer to the imaginary disc within which all the planets orbit.

So earth's obliquity to the ecliptic is 23 degrees, which is why we have seasons. Uranus's obliquity to the ecliptic is 89 degrees - it's lying on its side and days there last half the year.

So "on the same side of the ecliptic" would mean "on the same side of the sun".

If you drew a line from earth to the sun, then drew a line at ninety degrees to that line within the plane of our orbit, if Venus is nearer to us than that line, it's on the "same side of the ecliptic". (That was hard to put into words...)

Ah, yes you could call that the ecliptic, although it would usually be called the ecliptic plane. I'm not sure that venus is on the same side of the sun as the earth at the moment, because I looked at it the other night through the telescope and I couldn't see a crescent - it looked more gibbous to me, which would suggest it is on the other side of the sun.

Earth/sun/Venus angle right now is about 30 degrees.

http://www.fourmilab.ch/cgi-bin/Solar

Set to "inner system" to see easier.

Hi Gnomon & Hoovooloo - thanks for those comments, I'll try to take them on board next time.

Quick report for today.

14/3/2012 around 7:50 pm UK. Venus and Jupiter visible and very near to each other. As judged with respect to the Pole Star, Venus is still to the West of Jupiter, so should "cross over" to the Eastern side in a soonish. Mars highly visible in the East.

Around 11.10 pm UK. Finally some clear sky at this hour although some ground mist. Venus and Jupiter no longer visible so presumably have set, but Mars very clear, and surrounding stars forming Leo also clear. Mars appears to be in the middle of Leo. I follow the handle of the plough downwards to find a bright star (Arcturus) and follow that further to see Saturn and Spica moderately low in the Eastern sky. I am quite pleased to see Saturn - first time this year (and a long time before that).

Also see another bright star moderately low in the horizon in the north. My Planisphere tells me its Vega (I suddenly remember that's where the wormhole relay station was in the film Contact).

The past day or three the sky has been covered in altostratus / cirrostratus.

24/3/2012 7.10 to 8.25 pm evening
Clear sky, after sunset crescent moon observed in western sky (about 20 deg elevation), then from west to east, Jupiter, Venus and Mars. Mars is within Leo and Venus and Jupiter appear to be in the vicinity of Aries (I need to get myself a small pair of binoculars to supplement my naked eye observations).

Additional miscellaneous observations.
4 polar orbiting satelites.
One meteor - travelling north to south and passing through Leo.
Then something a little strange, which I later assumed to be Chinese sky lanterns:
Reddish-Orange glowballs appearing over the rise of the tree-topped Eastern horizon (behing which lie houses), moving swiftly "horizontally" (but probably rising) across the sky in a North-East to South-West direction and then disappearing over the tree tops on the South to South-West horizon. They were let off one after another in pairs and then a solitary one. I must check those upper air winds later on. I would guess the lanterns were about 100 - 250 m's high.

Later on.
Leo very clear with Mars inside it, Regulus is the brightest star in the constellation. Saturn very clear near to Spica which is the brightest star in the constellation of Virgo. Arcturus, Capella, Vega, the plough (overhead) all clearly visible.


Note1: Venus' maximum elongation is about 46 degrees (sun - earth - venus angle) so I am not sure whether that means Venus having "passed" Jupiter will "turn back" and repass Jupiter over the next few weeks or so as it heads back towards the Sun as viewed from Earth.

Note2: Wonders whether best time to see artificial satellites is an hour or two after sunrise or before sunset, as they are observed through reflected sunlight. Presumably in the middle of the night the satellites will be in the shadow of the Earth and hence not visible.

#44 Just noticed your link to the Solar system (I'll try and work out whats going on later)

Looking at that Solar System Live utility it seems that Venus is at more or less it's maximum elongation, which will now begin to shorten resulting in Venus repassing Jupiter over the next few weeks (as viewed from Earth).

25th March 2012, 21.45 to 22.35 (Brit. Summer Time = GMT+1)

Managed to see all at the same time four (plus one) planets in the sky (plus moon). From West to East - Jupiter (low in the western sky), Crescent Moon, Venus, Mars and Saturn (low in the eastern sky) plus of course the Earth. There seems to be a certain quality regarding the reflective light from Jupiter and Saturn (and Mars) that distinguishes them from the neighbouring stars.

Regarding the crescent moon: the crescent was on the western (sky)side of the moon and more along the northern side than the southern side (rather than symmetric around the local zenith). Also the "unilluminated" part of the moon was clearly visible, i.e., the complete disc of the moon was visible along with the crescent against the sky background. Presumably this was due to reflected "Earth" light.

Relative Position of the Crescent Moon: Slightly to the west and to the north of Jupiter. I would estimate that the elongations of the Moon and Jupiter were around 35 degrees with Venus at around 45 degrees. Presumably tomorrow the Moon will be next to Venus ("transiting" + 12 degrees of elongation in 24 hours).

Miscellaneous:
Whilst observing a polar orbiting satellite it "disappeared". I am not sure whether this was due to satellite rotation (to a non-reflective side) or whether the satellite passed into the shadow of the Earth (I suspect the latter).

"There seems to be a certain quality regarding the reflective light from Jupiter and Saturn (and Mars) that distinguishes them from the neighbouring stars."

Well, for a start, the light from any star appears as from a point source. The star is so far away that there is no discernible disc, and as a result all the light travels along the one path, and will be subject to 'scintillation', also known as twinkling.

Planets, on the other hand, are near enough that they appear as tiny discs. You might not see this, but the light rays from them will be spread out and will be less subject to twinkling, so unless the planet is very low down it will shine with a steady light.

I think that's what it is - stars twinkling whilst the planets don't. Also Saturn and Jupiter seemed to have the same paleish colour. I think Spica was more bluish and twinklish. It would be interesting to work out how many rods/cones individual stars/planets activate.

Twinkle Twinkle little star,
But not the planet as it's not too far.

26/03/2012 19.20 BST: Crescent moon to the south of Venus, about 4.5 moon diameters from it (2.25 degrees approx).

26/03/2012 22.25 BST: Again, all four planets and crescent moon visible, Jupiter, Venus, Mars, Saturn. Wonderful cloud free skies for two days now, one can see right to the horizon. I was observing the reddening Jupiter set and a bright meteor left a blazing trail, from North to South near to and along the Venus - crescent moon axis. It could have been an award winning photograph...

The crescent moon had moved slightly eastward of Venus compared to earlier (the moon moves about a full diameter eastwards compared to Venus over the course of an hour). Tomorrow I should imagine the moon will be near to but east of the pleiades.

I just have to record here the wonderful view I had of the moon tonight, while flying low into Manchester airport, the crescent moon was a strange orange shade, with whatever planet it was just above it to the right. It was odd being higher than the moon as it appeared slightly above the horizon.

That was Venus.

If you flew from Lanzarote to Manchester yesterday, it's unlikely the person I saw in Dublin on Sunday was you, then. This prosopagnosia is a curse!

It wasn't me, sorry. But I know what you mean about not mistaking strangers for people you know.

I said hello to a random Spanish lady, who just happened to have the identical hairstyle and body shape to a close friend of mine only the other day.

27/03/2012 22.30 BST: Jupiter sets. I had the impression it set a little later yesterday, but maybe I misremember. Another clear sky four planets etc. Crescent moon, near to but west and north of Aldebaran, main star of the Taurus constellation. Couldn't see the Pleiades.

Flying from Spain to the UK a few moons ago, clear sky, night-time, I was looking at the urban centres over France, all lit up. To the side I came across what appeared to be an oblong of a deep red neonish light, it turned out to be the moonrise (it was a fullish moon).

28/03/2012 22.27 BST: Jupiter sets. So Jupiter is setting earlier each day as the sun sets later. Also the angular distance between Venus and Jupiter seems to be increasing so it seems that Venus and Jupiter will not conjoin for some time. Perhaps Jupiter will pass behind the sun before Venus passes in front of the sun (transits) in early June.

Well the moon is more half moon now than crescent casting a clear shadow. The sky was clear again (four planets etc) but I had the impression the sky was a little "milkier", perhaps due to increased moonlight and atmospheric particles which one would expect more of for such weather conditions. The moon was between Aldebaran and Alnath (brightest and second brightest stars of Taurus. I reckon in two days time it will be next to Alhena in Gemini.

You have to remember that the Earth spins on its axis once every 23 hours and 56 minutes. So if Jupiter didn't move at all, it would set 4 minutes earlier each evening.