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Forum Index : Solar : equinox
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| isochronic Guru  Joined: 21/01/2012 Location: AustraliaPosts: 689 |
Just checking my simple solar astronomy .. Tomorrow (22 sept) is said to be the equinox - so at 12 noon the sun should be at its highest point for the day- and the sunlight's angle (from vertical) should be the observers latitude - and the sunlight should then range +/- about 26 degrees north/south over the year - is that all good ? Or is 21 sept better for Aus ? |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
From what I remember it is 21 Sept and about 26 deg sounds close enough, but it is 23.5 
George |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
Geez Stuart - now you've got me worried - I should know all that! I did a terrestrial navigation course (for sailing) back in the '60's. One night a week for three months with Norrie's Tables to the fore! Pity I can't remember pretty much any of it - about a year later we were all using RDF beacons instead in Bass Strait and now I'll bet there aren't many coastal sailors that can use anything but GPS. Greg |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Are you *really* sure about that? My understanding is that the sun is at its highest point *every* day at noon. By noon I mean noon corrected for longitude. AFAIK equinox means that the length of day & night is equal, that's all. The observers latitude can be determined with a sextant, horizon (at sea or artificial horizon on land) and correction tables, on every day at noon, noon being the exact time the sun is at the zenith of its arc. The sun can be directly overhead (90 degrees altitude) during the year between the topic of Cancer north of the equator and the topic of Capricorn to the south of it. No idea where you got your 26 degrees from as the topics are located at 23 deg 16 min N or S. Try this site to get your solar data. I would recommend further study of astronomy & navigation  . Klaus |
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| isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689 |
At equinox, the earth's axis should be perpendicular to the sun, so noon should be about right. The 23 degrees sounds better than my remembered 26; I think I saw the 26 somewhere as a figure that included vestigial light such as twilight etc. It is only an exercise, to estimate how sunlight will reach a site, as a scope for solar collection - not as important as navigation. It would be an impressive skill to take a position at noon with a sextant etc...But to get that accuracy, there are many factors and corrections, it is no wonder tables are used. Gimme GPS !!!! |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
Have to say your info sounds better Klaus - sorry Stuart Mind you, latitude is one thing, longitude another, as many old sailors learned the hard way.
The couple of times I did actually try to take some sightings at sea were a complete wipeout. The yacht bouncing around in Bass Strait made it pretty much impossible. The other fundamental thing was that you wanted your position mostly at night which meant stars, not the sun and fixing them was even worse! In the day you could generally at least see if you were about to run aground, night meant searching for those lighthouses - and making sure you got the correct sequence! I actually got to be pretty good at using charts at least. Should have given up offshore racing but it was a lot of fun - at least it was when I wasn't seasick! Gives you a whole new appreciation for the skills of those old navigators, doesn't it. Imagine being Cook and working your way up inside the Barrier Reef, no charts, big currents and only sails or warping to get you out of trouble. It's a miracle he didn't go aground before he did! Greg |
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| isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689 |
I should have said, aligned to north, (eesh). So, I guess (at equinox): at noon the sun is exactly north ? Is the correction for longitude still needed at that time ? |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Forget about that equinox thing - it appears to confuse you .
At local astronomical noon the sun is exactly North on *every* day in Australia. The sun is then at its highest point in the sky. But, as our time zones step in intervals of 15 degrees longitude, noon on your watch is only correct if you are located exactly on a longitude dividable by 15 degrees. If you are a few degrees east astronomical noon occurs sooner, a few degrees west and noon is later than what's shown on your watch. Since nobody likes to adjust their watch each time they move a little bit east or west from where they are the 15 degree longitude/ hour time zones have been established. Klaus |
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| isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689 |
Fair enough...I'll cogitate on it (and stick to solstices instead, and gps )
thanks all |
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norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Been following this thread with some interest, my take on it. If your south of the tropic of Capricorn the sun at noon is always to the north but if your north of the tropic of Capricorn the sun is south of you during "deep summer" at noon.ie when the earths tilt has the sun over the Tropic of Capricorn. This is very apparent when you have a ridge roof, with the ridge running E to W. Further north you are the greater the south angle the suns rays strike your north facing solar panels during summer, and your south facing wall is in the sun. That is why up here in NQ we usually have our fixed panels almost flat(but still tilted to the north) to get that summer sun, a compromise that works. If I`m wrong on this I`ll blame the Bundy rum, and scratch what hair I`ve left. We come from the land downunder. Vic |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
It is pretty simple, the equinox is when the sun is directly over the equator. There is no tilt of the earth in relation to the sun at all. At the solstice the sun is perpendicular to the surface of the earth, directly over the tropic of cancer or capricorn and the earth is at maximum tilt (23.4 degrees)north or south. The equinox happened this year at 6.44am Oz eastern standard time on Sept the 23rd. Calculating true day length on the other hand is a mind bending exercise and would take me many weeks and lots of futile hand gestures and pointless equations to explain, if you are interested Issac Asimov wrote an essay titled "Time is out of joint" that is published in a book "As far as human eye could see" that explains the difficulties of measuring time and a lot of other really interesting stuff that is part of our everyday life. I'm confused, no wait... maybe I'm not... |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks for pointing that out norcold. Myself, living at 31 degrees south, completely forgot about that important detail which is plainly obvious to you living in FNQ.
But what yahoo wrote about the earth's maximum tilt is unwilling to enter my head. As I understand it, the earths axis is tilted by 23.4 degrees from the plane around which the earth orbits the sun. This tilt remains the same (except for tiny variations) all year round. What causes the seasons and the apparent movement of the sun being directly overhead at noon between the tropics is the position of the earth on its orbit around the sun. In our summer the north pole points further away from the sun and in our winter its the south pole. This is probably best illustrated with two spheres (balls), the smaller one being held tilted and moved in an orbit around the bigger one. So the statement "the earth is at maximum tilt (23.4 degrees)north or south" is in my humble opinion not true. Klaus |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Think Yahoo2`s on the money. We wobble around a bit. Another titbit. Our summer solstice is moving 15m approx. north each year. I guess that should mean you folks down south will feel the cold more in say 100 years or so and we should cool a little. Tis an interesting subject for me as I have an observatory with a GPS equipped go to scope, but have not noticed any variation over the years, it still goes to accurately without realigning. We come from the land downunder. Vic |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
The reason that you wont see the difference is that astronomers calibrate from star time (sidereal day which is 23.9344696 hours) they don't care what time the sun comes up or goes down past the horizon, the cyclic eccentric movement is not even between each equinox and the next for each season but with simple programs to correct for this it is good enough for small telescopes. when you go big and start looking billions of light years distant, it matters a lot but atmospheric refraction is still a bigger problem to deal with. I'm confused, no wait... maybe I'm not... |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Was thinking along the line of the GPS side of the scope, with our real latitude varying by that 15m annually compared that which the GPS records( I think). Requires some georeferencing (eventually if Americans don`t recalibrate their sats ), of course I`m counting on finding that youth serum, thus hope to be around long enough to see the variation. We come from the land downunder. Vic |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
I've definitely learned something here gents - thanks for that. I'm also sure I'm not going to re-learn terrestrial navigation! Greg |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Just to clarify something, the earths axis it tilts on doesn`t change in relation to us, but in relation to the sun it does( my take of course). That is where I believe your thinking Tinker, remember the axis is through the poles which is perpendicular at the solstice`s and angled at the equinox`s in relation to the sun. Had heard about that 15m/year drift some time back and did some rough calculations, in 650,000 or so years if this drift continues, the poles will be at the equator. Checked on the net and the 15m/year figure is verified. Doesn`t sound right must be a flaw in my thinking here. More checking on the net is needed to determine if this drift is to continue. We come from the land downunder. Vic |
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Greenbelt Guru Joined: 11/01/2009 Location: United StatesPosts: 566 |
Hi Everyone; A little Drawing to add to "Tinkers" suggested experiment with the Spheres. Using a lamp to represent the Sun in a dark room, will shade the Earth Sphere for more Drama. As you move the Earth Sphere around the Sun (Lamp) be sure to always point the Pole to a fixed point in the room, A nail head, Picture, Or the Corner at 23 Degrees off perpendicular.to plane of Orbit. As noted in posts above, the Earth axis will over many many years change direction,(Gyroscopic Precession). Our lifetime will not be sufficient to notice a change. Cheers--------Roe 
 Time has proven that I am blind to the Obvious, some of the above may be True? |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
If you have a Android phone (Iphone should have a similar app) you can find where the sun rises and sets etc for any day of the year. The App I use is Google Sky Map, in the menu it has time travel set the date and time you are after. Point the phone in approximately the right direction, look for the sun on the screen of the phone. using this you can go anywhere and know what is going to be the best location for PV cells to avoid shading. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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