PDA

View Full Version : Jupiter too low in the sky?



sunrare
2011-Feb-19, 01:18 PM
About a month ago, I was walking in the early evening here in non-urban South East Australia (Very approximately 33deg South, 150deg East), and I'm pretty sure I noticed a planet in the sky (naked eye).

I assumed it was Venus as the Sun had been gone only about an hour, but a friend later looked up some astronomical site which implied it was probably Jupiter...

The worrying thing (for me) was, that this steady, unwinking, large light source was only about 20deg above the horizon, and if not due North, certainly no further West than North-West. (I thought Nor-Nor-West after checking my compass one time - I did allow for Magnetic North being about 12deg East of True North)

I thought all our naked-eye-visible Solar planets rose pretty much due East, travelled pretty much to the zenith and set pretty much due West - isn't that the Ecliptic?

What was I seeing?

Can Jupiter or any planet be that low in the sky AND that far from due West?

Centaur
2011-Feb-19, 06:19 PM
Welcome to the discussion group, sunrare. I really enjoyed my week in Sydney during 1969.

On 2011 JAN 19 as viewed from your location one hour after sunset, brilliant Jupiter was at an altitude of 24.6 above the horizon and at azimuth (direction) 285.4, which is 15.4 to the north (right) of due west. That is a geographic azimuth and not a magnetic one. Jupiter at magnitude -2.2 was about twice as bright as Sirius, the brightest fixed star, but substantially less bright than Venus. At that time and location, it was the only truly bright object in the section of your sky between due west and due north.

At your latitude, planets rise anywhere between east-northeast and east-southeast. They set anywhere between west-southwest and west-northwest. Between rising and setting they transit your northern meridian (line between due north and the zenith) at various altitudes but never reaching the zenith.

The celestial equator is the projection of the Earth’s equator onto the celestial sphere (sky), and at your latitude it extends from due east on the horizon, through due north at an altitude of 57, and then to due west on the horizon. The ecliptic is the intersection of the Earth’s orbital plane with the celestial sphere. It is inclined by 23.44 to the celestial equator. The ecliptic and celestial equator intersect in the constellations Pisces and Virgo. In Taurus and Gemini the ecliptic is more than 20 north of the celestial equator. In Scorpius and Sagittarius the ecliptic is more than 20 south of the celestial equator. By definition the Sun is essentially always on the ecliptic. The Moon and planets never stray far from the ecliptic. Jupiter is currently in Pisces.

I suspect that the object you saw was closer to due west than you imagined. Or the date or time of day were earlier than you remembered. Nevertheless, the object was almost certainly Jupiter.

sunrare
2011-Feb-21, 12:57 AM
Thanks Centaur, very informative. So can I just clarify your answer points?

So, none of the naked-eye-visible planets ever reach the zenith? So, in the past, when I thought I was naked-eye observing planets at the zenith, I couldn't have been? I.e., they must have been stars?

When you say the Sun is essentially always on the ecliptic, does that mean the ecliptic moves through the seasons? (The Sun is well off the zenith, to the North here in our Winter. We even notice it in Autumn & Spring.)
(But in our Summer, the Sun does reach the zenith.)

When you say, "The Moon and planets never stray far from the ecliptic.", I would've thought 24.6deg elevation and 15.4deg to the North of due West was well off the ecliptic?

It's possible that the date may've been earlier. What's the furthest to the North (deg from due West) that Jupiter can be at that elevation from my position (at any time of the year, but especially recently)?

Thankyou again Centaur, very informative. I look forward to your replies to this - that is, if you have the time.

CJSF
2011-Feb-21, 03:07 AM
It is very common to perceive objects as being at the zenith when they are really not. I find that when I look truly at the zenith, it feels like I am bending over backwards somewhat.

If Jupiter was somehow "too low" all of a sudden (or it was "gone" and some other bright celestial object appeared in a different location) I am pretty sure it would be world news and this blog would have been buzzing about it.

CJSF

Centaur
2011-Feb-21, 03:45 AM
Thanks Centaur, very informative. So can I just clarify your answer points?

So, none of the naked-eye-visible planets ever reach the zenith? So, in the past, when I thought I was naked-eye observing planets at the zenith, I couldn't have been? I.e., they must have been stars?

When you say the Sun is essentially always on the ecliptic, does that mean the ecliptic moves through the seasons? (The Sun is well off the zenith, to the North here in our Winter. We even notice it in Autumn & Spring.)
(But in our Summer, the Sun does reach the zenith.)

When you say, "The Moon and planets never stray far from the ecliptic.", I would've thought 24.6deg elevation and 15.4deg to the North of due West was well off the ecliptic?

It's possible that the date may've been earlier. What's the furthest to the North (deg from due West) that Jupiter can be at that elevation from my position (at any time of the year, but especially recently)?

Thankyou again Centaur, very informative. I look forward to your replies to this - that is, if you have the time.

You’re welcome, sunrare. You seem to be confusing three different celestial coordinate systems. The horizontal (alt-az) coordinate system is the one in which altitudes are measured from the horizon, and azimuths are measured around the horizon. That’s the system in which I told you the position of Jupiter on that evening. The equatorial coordinate system is based on the projection of the celestial equator onto the celestial sphere. It’s the one used to position telescopes with clock drives. The ecliptical coordinate system is based on the projection of the Earth’s orbital plane onto the celestial sphere. The ecliptic plane intersects the equatorial plane at an angle of 23.44. The Sun always appear on the ecliptic. The Moon and planets hug fairly close to the ecliptic. I suggest you thoroughly learn the differences among those coordinate systems by researching them on the internet. Here's a link to the related article in Wikipedia: http://en.wikipedia.org/wiki/Celestial_coordinate_system

The Sun never actually reaches the zenith (90 altitude) from your location. For latitude S 33 the Sun gets as high as 80.4 during the December solstice. That may seem close to the zenith, but it’s actually about 19 solar diameters away from it. By definition you’d have to be on the Tropic of Capricorn for the Sun to be on the zenith during the solstice. The Moon and planets could have gotten closer at times, but would never have quite reached the zenith.

Please reread the description in my previous post of how the ecliptic is inclined relative to the celestial equator, or go to sources that have diagrams. It’s not that the ecliptic moves, but that the Sun appears to move along the ecliptic (from the viewpoint of the orbiting Earth) causing the Sun to seem to oscillate annually relative to the celestial equator. The celestial equator appears stationary in your sky, although it's longitudes progress along it as the Earth rotates. The ecliptic appears to flutter daily due to the Earth’s rotation. Since the Earth rotates every 23:56 hrs while it takes 24:00 hrs for a point on the Earth’s surface to lap the Sun, these apparent oscillations shift position as the seasons progress.

Well recently that is exactly how far north Jupiter has been while at that elevation. It will be getting farther north during the coming years as its declination in the equatorial coordinate system becomes more northerly. If you go to my website you will find a chart I made to demonstrate Jupiter’s declination during recent and coming years. Sorry, it is designed for northern hemisphere observers, so you may want to copy it and turn it upside-down. It can be seen at http://www.curtrenz.com/jupiter

sunrare
2011-Feb-22, 11:28 AM
Thanks again Centaur. I knew the horizontal (alt-az - didn't know that term) coordinate system used angles of altitude from the horizon, and lateral degrees from one of the geographical points (I just used the 16 NNW, etc. descriptives) but I wasn't sure about the equatorial & ecliptic coordinate systems. I visited wikipedia as you suggested and am a little clearer now. Is there some site that has an graphic, animated instruction? I.e., to gradually zoom the viewer from a point in space, say, 67.56deg. off the ecliptic, down to their designated position on the Earth's surface, to see just how those diagrams of the Solar system we've all seen match with what we see in the sky?
Thanks again.

sunrare
2011-Feb-22, 11:41 AM
Thanks for your reply CJSF.

Yes, it feels to me also like I'm "bending over backwards somewhat" when I'm looking at the zenith.

As for your 2nd sentence, I didn't think I'd made some amazing discovery, I just always thought that the planets moved close to the zenith. For some reason, I've only recently started noticing planets (and the Moon sometimes) quite low in the sky - which, I guess from a lack of serious observation, I'd never noticed before. I was just trying to get my head around all those old diagrams of the Solar system (usually drawn from a point well off the orbital plane, looking "down" on it) with what I see in the sky. Somewhere along the way, I read, or was told, that the plane the planets orbit the sun in was visible to us as running from due East to due West and passing through the zenith. I do remember reading that not all planets' orbits lie exactly in that plane, but that greatest deviation was Pluto at about 20deg inclination, while the other planets deviated by only a few degrees. Is that right?

I guess my latitude moves things from the zenith, but I do remember seeing the Moon very high in the sky at times, and at others following a much more Northerly trajectory. It all seems so random, but I guess it's really just much more complex than I thought.

Centaur
2011-Feb-22, 05:02 PM
I do remember reading that not all planets' orbits lie exactly in that plane, but that greatest deviation was Pluto at about 20deg inclination, while the other planets deviated by only a few degrees. Is that right?


That’s essentially correct, although Pluto is not considered a major planet and its orbit is inclined 17 to the ecliptic. That extreme inclination is one of many factors that eventually caused the IAU to accept that Pluto needed to be reclassified.

The positioning of planets may seem random or complex, but a sense of order appears in one’s mind once the patterns become better understood. I suggest that you learn the zodiacal constellations and then keep a diary of the Moon’s daily position among them, also noting its horizontal coordinates. The Moon moves through the zodiac much faster than the Sun and planets, thus allowing the process to be learned more quickly. That’s what I did more than a half century ago when I was first learning how to observe celestial objects.

CJSF
2011-Feb-23, 03:47 AM
Also, the Moon is inclined 5 degrees or so from the ecliptic, making it run even higher (or lower) at times.

CJSF