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Paul Beardsley
2004-Nov-23, 08:46 PM
The other night I had a dream that I was travelling forward in time at some speed. I looked up at the sun, and instead of it arcing across the sky (as in H.G. Wells' The Time Machine) it was moving around a small section of the sky, describing an infinity sign (i.e. a figure 8 on its side).

When I woke up, I wondered if the situation I had dreamed was possible. If the planet I was on was tidally locked, but in an elliptical orbit, there would be libration causing the sun to wobble in the sky in an east-west direction. If the planet was tilted (as most are) the movement from solstice to solstice would provide a vertical component.

Can anyone confirm if the infinity sign is a possibility, or would it look more like a human eye?

ToSeek
2004-Nov-23, 09:07 PM
If your time machine were giving you periodic snapshots at the same time of day, you could have been seeing an analemma. (http://www.uwm.edu/~kahl/Images/Weather/Other/analemma.html)

Evan
2004-Nov-23, 09:08 PM
I don't recommend eating spicy foods just before bedtime.

Bozola
2004-Nov-23, 09:49 PM
If your time machine were giving you periodic snapshots at the same time of day, you could have been seeing an analemma. (http://www.uwm.edu/~kahl/Images/Weather/Other/analemma.html)

Don't you mean Ana Lemming (http://www.taiga.net/wmac/consandmanagementplan_volume3/graphics/photos/lemming.jpg)?

Paul Beardsley
2004-Nov-23, 10:11 PM
Thanks for your replies, everyone.

ToSeek - very interesting! It's too late at night for me to work out if the analemma would occur for a tidally-locked world, but I can't help thinking I was on the right track.

Evan - surely dream astronomy justifies spicy food late at night? Has anyone else had a similar experience? Perhaps someone had a vindaloo and dreamed up a plausible explanation for dark matter?

Bozola - very cute!

pghnative
2004-Nov-23, 10:22 PM
When I woke up, I wondered if the situation I had dreamed was possible. If the planet I was on was tidally locked, but in an elliptical orbit, there would be libration causing the sun to wobble in the sky in an east-west direction. If the planet was tilted (as most are) the movement from solstice to solstice would provide a vertical component.

Can anyone confirm if the infinity sign is a possibility, or would it look more like a human eye?Well, another way to ask this question is to speculate on how earth's path is traced in the lunar sky. Since lunar libration is only a few degrees, the earth may not trace an exact infinity sign but it might be close.

Squink
2004-Nov-23, 10:35 PM
Martian Analemma (http://antwrp.gsfc.nasa.gov/apod/ap030626.html)

Evan
2004-Nov-23, 10:50 PM
Hmmm. Must think some more.

Not simple at all for the moon.


The Earth and the Moon form in fact a "binary planet": each one is more closely tied to the Sun than to the other. The plane of the lunar orbit maintains an inclination of 5.145 396 with respect to the ecliptic (the orbital plane of the Earth), and the lunar axis of rotation maintains an inclination of 1.5424 with respect to the normal to that same plane. The lunar orbital plane precesses quickly (i.e. its intersection with the ecliptic rotates clockwise), in 6793.5 days (18.5996 years), because of the gravitational influence of the Earth's equatorial bulge. During that period, the lunar orbital plane thus sees its inclination with respect to the Earth's equator (itself inclined 23.45 to the ecliptic) vary between 23.45 + 5.15 = 28.60 and 23.45 - 5.15 = 18.30. Simultaneously, the axis of lunar rotation sees its tilt with respect to the Moon's orbital plane vary between 5.15 + 1.54 = 6.69 and 5.15 - 1.54 = 3.60. Note that the Earth's tilt reacts to this process and itself varies by 0.002 56 on either side of its mean value; this is called nutation.

http://en.wikipedia.org/wiki/Moon

For your hypothetical planet I suppose a figure eight is possible. Seems it would depend on the orientation of the axial tilt in respect of the apoapsis.

George
2004-Nov-24, 02:07 AM
If your time machine were giving you periodic snapshots at the same time of day, you could have been seeing an analemma. (http://www.uwm.edu/~kahl/Images/Weather/Other/analemma.html)

Don't you mean Ana Lemming (http://www.taiga.net/wmac/consandmanagementplan_volume3/graphics/photos/lemming.jpg)?
:P Not what I expected.

How 'bout ... Fleming (http://pblmm.k12.ca.us/projects/discrimination/Women/special/peggyflem.html)

ngc3314
2004-Nov-24, 03:12 AM
The other night I had a dream that I was travelling forward in time at some speed. I looked up at the sun, and instead of it arcing across the sky (as in H.G. Wells' The Time Machine) it was moving around a small section of the sky, describing an infinity sign (i.e. a figure 8 on its side).

When I woke up, I wondered if the situation I had dreamed was possible. If the planet I was on was tidally locked, but in an elliptical orbit, there would be libration causing the sun to wobble in the sky in an east-west direction. If the planet was tilted (as most are) the movement from solstice to solstice would provide a vertical component.

Can anyone confirm if the infinity sign is a possibility, or would it look more like a human eye?

My brain is slowing down for the night, but I think you could get an analemma in the shape of a (somewhat asymmetric) infinity symbol elongated E-W if the planet had a moderate axial tilt but synchronous rotation (however unlikely that combination may work out to be) and large orbital eccentricity, with the solstice points aligned with the ap- and perihelion, err, apo- and periastron, err, apo- and periapses...