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51 votes
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Why won't the Sun set for days at N66.2 which is below the arctic circle?

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 ...
notovny's user avatar
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36 votes
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JPL Horizons sending too many emails

It turns out the "more robust" mail queue manager implemented April 21 has a condition where it can rediscover itself. That condition occurred in the middle of the night local time with your ...
Jon Giorgini's user avatar
28 votes
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Why does the Earth have a z-component at the start of the J2000 epoch?

You made the same fundamental mistake that Anton Gromov made in his question on the sister Space Exploration StackExchange network site: You used the solar system barycenter rather than the Sun as the ...
David Hammen's user avatar
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23 votes
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How do planetarium apps and software calculate positions?

There are quite a few different types of methods of computing the position of celestial objects, and the method used to compute the position generally depends on the type of object and how accurate ...
Greg Miller's user avatar
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22 votes

Why won't the Sun set for days at N66.2 which is below the arctic circle?

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 ...
David Hammen's user avatar
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16 votes
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JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?

The JPL ephemerides are good, but they certainly are not precise to the fraction of a millimetre! From the Horizons docs: Statement of Ephemeris Limitations To produce an ephemeris, observational ...
PM 2Ring's user avatar
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14 votes

Why is the time between equinoxes different?

This is because the summer and winter solstices (approx. June 21st and December 21st) do not correspond to the aphelion and perihelion (approx. July 5th and January 4th). Therefore, the average ...
Glorfindel's user avatar
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12 votes
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What unit of year in JPL Small-Body Database Lookup?

Based on the Astrodynamic Parameters page, It appears to be the astronomical Julian year, defined as exactly 365.25 days of 86400 SI seconds.
notovny's user avatar
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12 votes

How do planetarium apps and software calculate positions?

How do planetarium apps and software calculate positions? The generic answer is that they use some form of ephemeris. The VSOP87 is an example of an ephemeris. So is JPL Horizons, which uses ...
David Hammen's user avatar
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11 votes

JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?

@PM2Ring's answer does a great job of explaining that folks who do careful numerical integration keep a large number of digits beyond the final accuracy because if they didn't, after tens of thousands ...
uhoh's user avatar
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10 votes

How to calculate the true distance of the sun and the earth using skyfield?

Your procedure is sound! Here is the same calculation, using Skyfield's ability to build a vector of all the hours of the year 2030, and NumPy's ability to find where the maximum and minimum occur in ...
Brandon Rhodes's user avatar
9 votes

Why won't the Sun set for days at N66.2 which is below the arctic circle?

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 ...
Chappo Hasn't Forgotten's user avatar
9 votes
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Earth-Moon Barycenter Perihelion

From a comment by the OP, How do I set it to the center of the Sun? Select Vector table as the ephemeris type. Choose the Earth-Moon barycenter as the target body and @sun as the coordinate center. ...
David Hammen's user avatar
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7 votes

Instantaneous gravity in equation of motion for ephemerides calculation

Would this not mean that information travels infinitely fast and thus contradict Relativity? Look more closely at equation 27 in the referenced document. I'll simplify this as $$\boldsymbol{\mathrm a}...
David Hammen's user avatar
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7 votes
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Question about Cowell's method

Yes, that interpretation is correct. The given formula only denotes the (integral) acceleration seen by a single mass element at a certain time $t$ (as implicitly $\mathbf{r_i} = \mathbf{r_i(t)}$ ...
planetmaker's user avatar
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7 votes

How to get JPL Ephemeris data to work in n-body simulation or why is Mercury flying to Jupiter

I used this n-body algorithm. Do not use that algorithm, repeated many times over. As a starter, the algorithm uses symplectic Euler. Paraphrasing from the movie "Jaws", "You're going ...
David Hammen's user avatar
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7 votes
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Different results for the same JPL Horizons query

The issue is with differences in how the defaults behave between the code and the website. In the code version of the API id=3 gives the Earth-Moon Barycenter (as ...
astrosnapper's user avatar
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7 votes

How to test accuracy of DE441?

I'll start at the end and walk your question backwards to provide a partial answer. However I too am looking forward to find out if there is any independent testing of the DEs using phenomenon not ...
uhoh's user avatar
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6 votes

Apsides Calculation

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 ...
Mike G's user avatar
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6 votes
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Accuracy of calculating the vernal equinox?

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: $$ \...
Mike G's user avatar
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5 votes
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L2 point ephemeris (celestial mechanics)

Bad news, this type of SPK file has a different sort of interpolation that is not supported by the jplephem package (Hermite interpolation vs Chebyshev polynomials)...
astrosnapper's user avatar
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5 votes
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Why is the time between equinoxes different?

The diagram below illustrates the reason, which is that the earth moves at different speeds during it revolution around the sun and the distribution of those speeds is not equal because the solstices/...
Tyler Durden's user avatar
5 votes
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Predicting accurate time of local sunrise in mountainous area?

The PeakFinder app1 can do most of what you're looking for. It's available for Android and IOS as well as through a web browser. The app shows the paths across the sky for both the sun and moon2 and ...
Alex Hajnal's user avatar
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5 votes
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Implementing the VSOP-2000 Ephemeris

Probably the most reliable guide to vsop2000-p11.dat is the reference implementation vsop2000.for. The input line in question is ...
Mike G's user avatar
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5 votes

Instantaneous gravity in equation of motion for ephemerides calculation

Short answer is "no". I can model gravity by Newton's law of gravitation, and it gives extremely accurate results in most situations. Even though it is an approximation to the more accurate ...
James K's user avatar
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5 votes

Is there any ephemeris generator that gives you accurate distances between Earth and other bodies?

JPL HORIZONS gives the distance from the observer to the target as delta. In Table Settings this option is 20. Observer range & range-rate. For asteroids and ...
Mike G's user avatar
  • 18.7k
5 votes

Apsides Calculation

Do you need 1000 years? 50 years? 1-hour accuracy? 1-second accuracy? A simple linear interpolation for the perihelia from the year 2000 to 2050 gives a maximum error of about 1.3 days for the year ...
Cristiano's user avatar
  • 256
5 votes
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Linear ephemeris

This is a fairly standard ephemeris for e.g. times of maximum light of a variable star but you are getting some of the symbols mixed up. The ephemeris is normally written as: $$ T_{event} = T_0 + P\...
astrosnapper's user avatar
  • 8,357
5 votes
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Resolving variance between Skyfield-generated sun vectors and test data when both use same ephemeris

Let’s try narrowing this down to a smaller test case, to make the problem tractable, and ask why the vector isn’t looking right for one particular time. I often find that much easier than trying to ...
Brandon Rhodes's user avatar
4 votes

How can we map masses orbiting the outer Solar System by determining the orbits of planets and smaller objects?

Iorio (Preliminary constraints on the location of Telisto/Planet Nine from planetary orbital dynamics) has suggested transmissions from the New Horizons probe (currently beyond Pluto) could be used to ...
Andy's user avatar
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