# Questions tagged [ephemeris]

Questions about the predicted positions of an object in space, like another planet, comet, or asteroid.

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### 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. (Yes if you ...
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### JPL Horizons sending too many emails

I submitted a job that, I think, should have returned 100 emails from JPL Horizons. I now have over 6000 emails and they are coming in at about four a minute. Anyone else had this problem? Anyone know ...
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### Why does the Earth have a z-component at the start of the J2000 epoch?

When I go to the NASA HORIZONS webpage https://ssd.jpl.nasa.gov/horizons.cgi and ask for the VECTOR coordinates of the Earth on 1 January 2000, it tells me that Earth has the following coordinates ...
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### How do planetarium apps and software calculate positions?

In the most general terms (I'm not asking for the actual calculations), how do popular planetarium apps and software calculate the positions of celestial objects? Planets, for example. Do they use ...
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### JPL Horizons - "highly accurate measurements of planetary positions" - how do they do it?

According to JPL Horizons, with regard to their planetary position calculations, ... we integrate the equations of motion in Cartesian coordinates ($x,y,z$), and we adjust the initial conditions in ...
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### Why is the time between equinoxes different?

The path of the Earth around the sun is described as being an ellipse, with the Sun being at one of the foci of the ellipse. Since an ellipse, by definition, is symmetrical with respect to the foci, I ...
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### What unit of year in JPL Small-Body Database Lookup?

I'm new to astronomy and therefore probably missing the obvious here, but what type of year (tropical, sidereal?) is used for the orbital periods in the JPL Small-Body Database Lookup? For example, ...
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### Apsides Calculation

I’m trying to calculate the precise moment of Earth’s apsides (perihelion and aphelion). The only formulas I find are in Astronomical Algorithms by Jean Meeus (1998). However, they give erroneous ...
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### Earth-Moon Barycenter Perihelion

I am trying to implement an algorithm I created to get the sun rise, transit and set accurately, but in order to do that I need to get the Earth-Moon barycenter (EMB) perihelion, and not the Earth ...
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### What are the problems that need to be taken in consideration when trying to find planetary position by using jpl ephemerides?

I'm currently trying to determine the position of the sun and the moon from jpl ephemeris DE200 which is referred to the dynamical equator and equinox of 2000 and uses tdb time. I am using the j2000 ...
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### Exactly what in Saturn has been located to within 4 km precision?

Saturn has been located with fantastic precision thanks to Cassini and VLBI. The location of the 120,000 km diameter giant planet (when defining the 1 bar pressure level as its surface) is known to ...
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### Formula to Determine When a Target Reaches a Specific Altitude

I'm trying to develop an Excel macro to compute when a target will rise above a specified altitude given the date and location. I can get the correct answer with Stellarium and using a Python program ...
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### Beginners question about time scales

I'm using https://github.com/Bill-Gray/jpl_eph for a personal project to read JPL DE files, namely DE430 and the more recent DE440. This generally works great and when tested against a reference ...
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### Understanding the JPL Ephemerides

I'm working on an animation that replicates our Solar System. I know this has been done before, but I'm working on something that requires me to make one myself. With that being said, I'm having a ...
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### Different results for the same JPL Horizons query

On JPL Horizons website, I'm using the following query to get vector state for Earth: ...
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### Uncertainities of orbital elements queried from JPL Horizons

I'm trying to retrieve the orbital elements' uncertainities for asteroids queried from NASA JPL Horizons. However I'm not able to find them in the query fields returned in python. Nor am I able to ...
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### Where to get rate of change for calculating ephemeris from JPL Horizons

I have a working implementation of Schlyter's method for calculating the Sun's position. I would like to know how Schlyter got the 3.24587E-5_deg in the following calculation for Mercury. ...
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### JPL DE Documentation

I'm currently trying to write a C++ library that makes accessing binary data from a JPL ephemeris file easier. So far I sifted through the web, trying to find a good and thorough documentation about ...
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### Discrepancy between my calculated planet JPL ephemerides and those by Horizons online app

I'm currently using JPL DE440 ephemerides. Following the documentation here, verified with the header file (specially the records in GROUP 1050) for the DE440 ephemerides (file header.440 available ...
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### Which kernel does HORIZONS get the Geometric Albedo from?

I am new to using SPICE and the SPICE Toolkit. My goal is to extract body radius, albedo, and distance from the sun of a number of solar system objects. The tools provided by NAIF are great and I ...
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### CSPICE vs JPL Horizons discrepancy in AZ/EL

This CSPICE spkcpo_c routine example outputs: ...
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### How can I calculate my position (lat/lon) given a sequence of solar-elevation measurements at known times?

The "forward" problem (given time and lat/lon, find solar position angles) is well documented, e.g. here. I am trying to figure out the "inverse" problem: given time and [a ...
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### How accurate should sun\moon ephemeris be for calculating eclipses?

I am writing a program to simulate eclipses for about -2000 to 6000. My question is, how accurate does my ephemeris for the sun and moon have to be to get a decent accuracy for eclipses? I currently ...
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### Precision in the measurement of the distance to the Sun

How precisely can the distance to the Sun be measured? Wikipedia says the distance to the Moon can be measured upto millimeter precision. But Wikipedia article on distance to the Sun says only about ...
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### Trying to find the source of these three orbital elements

I'm working through section 65 “Calculating the Moon's position” in Practical Astronomy with your Calculator or Spreadsheet (fourth edition) by Duffett-Smith and Zwart. They begin the calculations ...
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### How to test accuracy of DE441?

I want to write about processing DE441 data for Moon and Sun and include a section about the validation and accuracy testing of the algorithm. Is there a website where I can retrieve documentation for ...
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### How to get JPL Ephemeris data to work in n-body simulation or why is Mercury flying to Jupiter

I wanted to make a n-body simulation for an art project. I used this n-body algorithm, which includes a stable system for the Sun + gas giants, but no inner planets. I wanted to simulate inner planets ...
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### Calculating velocity of Earth from JPL DE

We have discussed how to calculate positions from JPL development ephemerides in this question. Following this answer, I am now using SPICE's algorithm for calculating the derivatives of the Chebyshev ...
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### Linear ephemeris

What is the T in the formula for the linear ephemeris T_max = T + P*E, please? P stands for the period; E, for ephemeris. How to determine T? I have T_max and P. Or is it a referent epoch? Many thanks
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### Predicting accurate time of local sunrise in mountainous area?

Is there any software which will tell me accurately at what moment any part of the solar disk will become from a specific point on the ground taking account of terrain. I feel like Google maps should ...
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### Calculation of Terrestrial and Universal time scale difference, ΔT

$\Delta$T is defined as the 'measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of atomic time'. I tried to calculate $\Delta$T corresponding to ...
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### For how many solar system bodies do recent JPL Development Ephemerides include libration?

Since the Earth and the Moon are so gravitationally and tidally engaged and the laser ranging data so precise, the JPL Development Ephemerides are based on extensive modeling of the Earth-Moon system ...
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### How to calculate the ground track of the Moon's position on the Earth's surface?

I would like to project the Moon's position on to the surface of the Earth, starting with its position expressed as Right Ascension in degrees. I know that the declination of the Moon corresponds to ...
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### How can we map masses orbiting the outer Solar System by determining the orbits of planets and smaller objects?

Neptune was discovered 170 years ago with the help of observations of Uranus' orbit. A great demonstration of the scientific principle for its time. Recently, Earth and cis-Lunar based radio ...
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### Determining moonset time from Nautical Almanac

I have recently learnt to determine moonrise and moonset times using the Nautical Almanac, as part of my maritime navigation course. I have a query regarding the determination of moonset times. I will ...
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### Difference in Ephemerides Position:- PlanEph 4.2 (VSOP87) vs JPL Horizons (DE431)

I have obtained the hindcast position of Mercury relative to the Sun for the epoch 2000-Jan-01 12:00:00 using two methods: PlanEph ver 4.2 (Dos-based) and JPL Horizons DE431 (online). The two methods ...
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### How to calculate the point on the globe with maximum magnitude with given Besselian elements and given time for a partial solar eclipse?

I am currently working on improvement of an Android app that calculates solar eclipses. For many days now I have not been able to solve a mathematical problem concerning solar eclipses, so I decided ...
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### How to visually chart HORIZON ephemeris

I'm trying to build a 3D model of the Sun, Earth and Moon in in Javascript (using Three.JS, which relies on x,y,z coordinates for all objects) in order to simulate the Aug. 21 eclipse. I've ...
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### Accuracy of calculating the vernal equinox?

Calculating the exact time of the vernal equinox is essential for many ephemeris calculations. NASA Goddard Institute for Space Studies uses (at least for some purposes) the following, rather short ...
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### How to determine when Sun/Moon is aligned to two location on Earth's surface?

I would like to make a photo to Moon/Sun when it is very low on horizon, so if I make the shot far away from an object on Earth surface, it appears as big as the Sun. But how can I calculate the right ...
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