Expert: Tom Whiting - 9/29/2009

Question
I am try to collect data to visulize the motion of polaris as viewed from a latitude of zero over a one year period. in example if this was done as a time laps photo would form a circle? or a figure eight? or an out of balanced figure eight?
I saw Edward Leedsklanin sun dial at coral castle Homestesd Fl @ 25deg30'02.86" N / 80deg26'39.54" w trying to understand how it works. thx Valery

Answer
Hi Valery,
You collect data by traveling to the equator (latitude zero) and viewing Polaris for yourself....
what you are actually doing here is collecting opinions from experts, rather than traveling
to the equator to collect your own data.

Since Polaris is not exactly at the true pole (It's currently 0.7 degree from the pole)
it describes a small circle in our sky with a diameter of about 1.5 degrees (3 apparent
diameter full moons) in our 24 hour rotation sky.
So at zero latitude (equator), an observer in total darkness for 24 hours would theoretically see Polaris rise at azimuth 359.3 degrees, reach a maximum height of 0.7 degree at azimuth 360 or 000 degrees 6 hours later, then set at azimuth 000.7 degrees 6 hours after that. Thus describing a small semi-circle on the northern horizon for 12 hours. Then for 12 hours, it would be below the observers northern horizon (It's elevation would be -0.7 degree, below your
horizon).

But since there is not 24 hours of darkness, there are only certain times of the year that
this would occur during the observer's 12 hours of darkness.  Polaris has an upper culmination
and a lower culmination. An observer at the equator would only see Polaris during their
night-time hours when Polaris is at or near it's upper culmination (highest in sky) at midnight local time. This occurs on or about November 1st.  So around that time, an equatorial observer would see Polaris rise at about 6 pm, reach its highest elevation of 0.7 degree at midnight, and
set around 6 am.  On May 1st, lower culmination of Polaris occurs, so an equatorial observer
never sees Polaris in their night sky as it's below their horizon all night, and above their
horizon all day long...invisible to them. At all the other times of the year, Polaris would
be partly visible throughout the hours from dusk to dawn.  For instance on August 1st, Polaris
would rise around midnight and reach it's highest elevation of 0.7 degee at dawn.
On Feb 1st, Polaris would be at it's highest at 6 pm dusk, and set at midnight.

The sundial time has nothing to do with Polaris' visibility or small circle in the northern sky...sundial shape pertains to our elliptical orbit around the sun and produces a figure 8 shape due to the Earth's variable speed around the sun of an ellipse. Polaris semi-circle on the northern horizon at the equator (during the night time hours) is strictly because of our 24 hour rotation, and the fact that Polaris is 0.7 degree from the true pole.

Biologists take "time lapse" photos....say one picture of 1/40 second every minute for say
6 hours of a stationary flower. It shows flowers opening up very quickly when played back. That's time lapse photography.  Astronomers don't do that...they take long time "exposures" keeping the camera lens open for say all 6 hours. As the earth turns, all the stars 'trail' across the film. Even Polaris 'trails' a small amount because it's not at the true pole.
So a one night time exposure of Polaris would show a semi-circle on the northern horizon from
the equator....or during the entire year, you would see Polaris at the same time of night, you
would see Polaris above your northern horizon for 6 months, and it would be below your horizon
for 6 months under theoretical perfect conditions. This is time exposure photography, not time
lapse photography.
In reality, with atmospheric refraction, clouds, haze, atmospheric extinction near the horizon...
Polaris would be reduced in brightness by 3 or 4 magnitudes, from a 2nd magnitude object to
a 5th or 6th magnitude star, barely visible to the naked eye. In addition, the pinpoint image
would be swamped with horizon haze at less than one degree high, so it would be a faint blob
(non-pinpoint) of light instead of a sharp pinpoint most of the time.
So that's how it would work.
Clear Skies,
Tom Whiting
Erie, PA