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Date: 28 Aug 2006 22:20:32
From: David Gallermo
Subject: Effect of 90 degree axial tilt upon apparent motion of sun
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I've been wondering about what sunrises and sunsets would be like if
the Earth had an axial tilt of 90 degrees. So far, I've figured out
that during summer solstice, the north pole would see the sun
motionless at zenith all day, and gradually spiral down to the horizon
by late September before disappearing until March, and then spiraling
up into the sky until it reached the zenith again in late June.
Likewise, a spot on the equator would see equal days and nights, with
sunrise and sunset all along the eastern and western horizons, except
for the solstices, when the sun would hang motionless on the north or
south horizon (north in June, south in January).
My problem is trying to figure out what the sun's apparent motion would
be for an observer at, say 45 degrees north. All I've been able to
figure out is that in late June, the sun would trace a circular path
around the sky at a constant angle of 45 degrees above the horizon;
sometime in November the sun would appear only briefly at the southern
horizon for a few moments around noon before disappearing for two
months, reappearing at the southern horizon again about noontime in
early February.Then in late March, the sun would reach an angle of 45
degrees above the southern horizon at noon. But oddly enough, come
early May, when the location reached its "nightless" phase, the sun
would touch the northern horizon at midnight, then swung upward and
eastward until by noon it was at zenith.
This I find curious: from then until the solstice, would the sun's
maximum angle above the southern horizon at noon actually decrease to
45 degrees by solstice? Somehow it seems counterintuititve. Am I
visualizing this correctly, or am I on the wrong track?
Thanks for any help you can give me.
Sincerely,
David Gallermo
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Date: 29 Aug 2006 08:51:46
From: Bill Owen
Subject: Re: Effect of 90 degree axial tilt upon apparent motion of sun
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David Gallermo wrote:
> I've been wondering about what sunrises and sunsets would be like if
> the Earth had an axial tilt of 90 degrees. So far, I've figured out
> that during summer solstice, the north pole would see the sun
> motionless at zenith all day, and gradually spiral down to the horizon
> by late September before disappearing until March, and then spiraling
> up into the sky until it reached the zenith again in late June.
> Likewise, a spot on the equator would see equal days and nights, with
> sunrise and sunset all along the eastern and western horizons, except
> for the solstices, when the sun would hang motionless on the north or
> south horizon (north in June, south in January).
>
> My problem is trying to figure out what the sun's apparent motion would
> be for an observer at, say 45 degrees north. All I've been able to
> figure out is that in late June, the sun would trace a circular path
> around the sky at a constant angle of 45 degrees above the horizon;
> sometime in November the sun would appear only briefly at the southern
> horizon for a few moments around noon before disappearing for two
> months, reappearing at the southern horizon again about noontime in
> early February.Then in late March, the sun would reach an angle of 45
> degrees above the southern horizon at noon. But oddly enough, come
> early May, when the location reached its "nightless" phase, the sun
> would touch the northern horizon at midnight, then swung upward and
> eastward until by noon it was at zenith.
>
> This I find curious: from then until the solstice, would the sun's
> maximum angle above the southern horizon at noon actually decrease to
> 45 degrees by solstice? Somehow it seems counterintuititve. Am I
> visualizing this correctly, or am I on the wrong track?
>
> Thanks for any help you can give me.
>
> Sincerely,
>
> David Gallermo
>
You've got it basically right. Consider the diurnal path of *stars* at
various declinations. At latitude 45 N, Ursa Major gets higher in the
sky than Polaris, so when the sun is at declination +45 (early May) it
would go through your zenith at noon and be tangent to the horizon at
midnight. As it continues north, the small circle it traces out would
shrink.
Some other things worth noting:
-- The Arctic Circle, Antarctic Circle and equator would coincide.
To put it another way, every spot on earth would experience a "midnight
sun" at some time during the year.
-- The sun would be at RA 0h from December until June and at RA 12h
from June until December. Successive transits of the sun would occur
(hoo boy, I bet this generates a response from someone) 23h56m04s
apart -- except that at the solstices, the sun's RA instantaneously
increases by 12h and you could get successive transits separated by
either 12 or 36 sidereal hours.
-- If the earth's orbit were circular, the rate of change of the sun's
declination would be constant, roughly 1 degree per day, either positive
or negative as the case may be.
Hope this helps. Interesting problem. :-)
-- Bill Owen
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Date: 31 Aug 2006 10:27:56
From: Don't Be Evil
Subject: Re: Effect of 90 degree axial tilt upon apparent motion of sun
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David Gallermo wrote:
> I've been wondering about what sunrises and sunsets would be like if
> the Earth had an axial tilt of 90 degrees. So far, I've figured out
> that during summer solstice, the north pole would see the sun
> motionless at zenith all day, and gradually spiral down to the horizon
> by late September before disappearing until March, and then spiraling
> up into the sky until it reached the zenith again in late June.
> Likewise, a spot on the equator would see equal days and nights, with
> sunrise and sunset all along the eastern and western horizons, except
> for the solstices, when the sun would hang motionless on the north or
> south horizon (north in June, south in January).
>
> My problem is trying to figure out what the sun's apparent motion would
> be for an observer at, say 45 degrees north. All I've been able to
> figure out is that in late June, the sun would trace a circular path
> around the sky at a constant angle of 45 degrees above the horizon;
> sometime in November the sun would appear only briefly at the southern
> horizon for a few moments around noon before disappearing for two
> months, reappearing at the southern horizon again about noontime in
> early February.Then in late March, the sun would reach an angle of 45
> degrees above the southern horizon at noon. But oddly enough, come
> early May, when the location reached its "nightless" phase, the sun
> would touch the northern horizon at midnight, then swung upward and
> eastward until by noon it was at zenith.
>
> This I find curious: from then until the solstice, would the sun's
> maximum angle above the southern horizon at noon actually decrease to
> 45 degrees by solstice? Somehow it seems counterintuititve. Am I
> visualizing this correctly, or am I on the wrong track?
>
> Thanks for any help you can give me.
>
> Sincerely,
>
> David Gallermo
That's essentially Uranus' situation.
Greg
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Date: 01 Sep 2006 11:53:08
From: Kevin Heider
Subject: Re: Effect of 90 degree axial tilt upon apparent motion of sun
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On 31 Aug 2006 10:27:56 -0700, "Don't Be Evil" <g626700-gg@yahoo.com >
wrote:
>
>David Gallermo wrote:
>> I've been wondering about what sunrises and sunsets would be like if
>> the Earth had an axial tilt of 90 degrees.
>> David Gallermo
>That's essentially Uranus' situation.
>
>Greg
This may not help the OP too much, but for those that are curious...
See Uranus as viewed from Earth 1994, 1997, 2006 at:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/42/image/d
-- Kevin Heider
West Coast Swing Photos at:
http://www.pbase.com/kheider
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