52

From the PyEphem Quick Reference Guide: Rising and setting are sensitive to atmospheric refraction at the horizon, and therefore to the observer’s temp and pressure; set the pressure to zero to turn off refraction. It seems likely that, if you're using the default settings, the result returned is including atmospheric refraction, giving the results you ...


22

Correct me if I am wrong, but if we count sunsets by the center of the Sun apparently crossing the horizon then the Sun is supposed to set every day at latitudes under the arctic circle. That is not how PyEphem defines sunrise and sunset. It defines sunrise as the time the top of the Sun would nominally first appear above an unobscured horizon (no mountains)...


9

Wikipedia's article on the Arctic Circle provides the explanation. Firstly, it says: because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the northern summer solstice up to about 50 minutes (′) (90 km (56 mi)) south of the Arctic Circle. As the Arctic Circle is currently at roughly 66°34′N, this means a ...


8

The motion of the Sun in the direction perpendicular to the Galactic plane is perfectly well understood. The mass of the stellar component, which dominates over dark matter at the solar galactocentric radius, is strongly concentrated towards the plane - that's why it is called the Galactic plane. Very roughly, you can characterise the density as $\rho_0 \exp(...


6

Nobody says that nothing exists "above" or "below" the galactic plane. The stars are thickest at the galactic plane and get scattered thinner and thinner with increasing distance from the galactic plane. It is often said that the disc of the galaxy is only 1,000 light years thick, but that is a round figure. The galactic halo is a ...


5

For those who don't have ready access to a copy of Astronomical Algorithms, Meeus's first approximation looks like: $$ \text{JDE} = 2541547.51 + 365.259636 ~k + 1.6 \times 10^{-8} ~k^2 $$ where k, the number of anomalistic years since the January 2000 perihelion, is half of some integer. This neglects the influence of the Moon and other planets, so he adds a ...


3

I wrote the source you need some years ago: https://jumpjack.wixsite.com/progetti/sorgenti-ipsun The Arduino/Processing version was just a demo program to manually control a TENVIS camera by multiple buttons, it lacks the "astronomical algorithm". The Javascript version contains astronomical calculations and a demo page which connects to a local ...


3

What Kepler (and others before and after him) wanted to do is predict where a planet would be. To do this we need some set up: First we want a coordinate system. This is a system of axes: x, y and z, at right angles to each other. And it should be an inertial coordinate system, so Newton's laws work. This means that the axes should not be rotating. And we ...


2

The climate modelers' VERNAL is equivalent to this formula, where Y is an integer, ΔY = Y - 2000, and JD0 is epoch J2000 = JD 2451545.0 = 2000-01-01 12:00 TT: $$ \text{JD}_\unicode{x2648}(Y) = \text{JD}_0 + 78.813 + 365.24250~{\Delta Y} $$ We can tune the coefficients to fit JPL DE431 for years -5000 to 9000: $$ \text{JD}_\unicode{x2648}(Y) = \text{JD}_0 ...


2

Calculating the exact time of the vernal equinox is essential for many astronomical calculations. I dispute that claim. What is true is that calculating the exact time of the vernal equinox is essential for some astrological and religious calculations. My question: Is the above quoted algorithm the most accurate way of calculating the vernal equinox for ...


2

Assuming you know Sun Altitude/Elevation and Azimuth at a given location on Earth (you can calculate it using any astronomy library, such as suncalc.js), and the Altitude/Elevation and Azimuth of target w.r.t mirror, the mirror must point toward this direction: mirrorAz = TargetAz + (SunAz - TargetAz) / 2 mirrorAlt = TargetAlt + (SunAlt - TargetAlt) / 2


1

The "dark" side of the Moon is only truly dark during full Moon. Everywhere on the Moon there is day and night as well. The dark side of the Moon is called like that because we do not see it from Earth, since the Moon shows us always the same side due to tidal locking - not because it is always dark there. In other words: During (lunar) day, the ...


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