Questions tagged [positional-astronomy]

Questions about positions of objects in the sky, either relative to a fixed coordinate system like the equatorial and ecliptic coordinate systems, or relative to an observer's point of view.

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How can I calculate the geocentric longitude of Pluto in a similar way to VSOP87's algorithm?

The book "Astronomical Algorithms" teaches a way to calculate the position of any of the planets for almost any date, until you get to Pluto. To try to get around that, I attempted to ...
0 votes
1 answer
88 views

What is orientation?

I'm trying to understand how SPICE software works, but I'm afraid that I need to understand a lot of astronomy concepts. One of the kernels I have use has this description: These kernels contain ...
4 votes
1 answer
130 views

Why is Eris not a part of the orbital plane like other planets?

I realise that the planets are more or less part of the same orbital plane. But why is the orbit of Eris, or even Pluto, at such significant tilts of orbit (44 degrees and 17 degrees respectively)? I ...
2 votes
1 answer
118 views

Relation of length of small circle with that of a great circle

This is not the best place to ask such doubts, so please recommend where I can. I currently don't have the facility to ask a teacher specialized for astronomy. So I just got reading- "Astronomy: ...
1 vote
1 answer
71 views

Transformation matrices and their derivation (if possible) for ICRS frame for reference to other frames

I am finishing up my thesis write up and one of the chapters in my thesis is on astronomical coordinate systems since in my analysis of data I have converted the available astrometry from the ICRS ...
0 votes
1 answer
43 views

What year will Gamma Cephei become the North Star?

I tried googling but I couldn't get any exact year, just any time between 3000 and 3100 CE. I want to know the exact year the north pole becomes closer to Gamma Cephei than Polaris. Apparently Polaris ...
5 votes
2 answers
355 views

Elevation difference of analemmas' taken at different hours and the reason for the tendency of analemmas' being slanted with deviation from the noon

According to Wikipedia, in astronomy, an analemma (/ˌænəˈlɛmə/; from Ancient Greek ἀνάλημμα (analēmma) 'support') is a diagram showing the position of the Sun in the sky as seen from a fixed location ...
7 votes
1 answer
170 views

Finding stars that would be visible from satellite

I'm working on a mock image of a picture taken from a satellite (like ISS). Lets say I have a camera on ISS pointing in the direction of velocity (I have this velocity vector defined in ITRF and ...
2 votes
1 answer
147 views

On what four dates dates do we observe the minimal, maximal, and two days of zero Solar axial tilt relative to us?

In case the title is a bit confusing, what I'd like to know is essentially respectively on which dates the Solar axial tilt is pointing straight towards Earth, straight away from Earth, and ...
1 vote
2 answers
438 views

Error when calculating Alt/Az from Ra/Dec

I wrote a program based on this tutorial: http://www.stjarnhimlen.se/comp/ppcomp.html to calculate altitude and azimuth of a celestial object. When I compare the calculated RA/Dec values to the ones ...
6 votes
1 answer
306 views

Geometric effect on sun's position in the sky at the equator

This question is about an explanation of "Sunrise and Sunset Times Near the Solstices" on the official website of the US Naval Observatory. It is mentioned in the explanation article that, ...
5 votes
1 answer
636 views

Difference in results between JPL Horizons and cspice (rust-spice)

I am calculating the apparent planetary position of planets using the cspice library (I am specifically using rust-spice, which is a wrapper of cspice in Rust). when I calculate the same values in the ...
4 votes
2 answers
135 views

Resolving variance between Skyfield-generated sun vectors and test data when both use same ephemeris

I have two functions that get TEME and GCRS Earth-Sun position vectors using Skyfield: earth_sun_vectors.py ...
2 votes
0 answers
87 views

Simple position calculation of solar system satellites

I'm looking for a simple way to calculate positions of satellites in the ecliptic of the date. I've found an implementation for Jupiters satellites in the Astronomical Algorithms by Jean Meeus, and I'...
0 votes
0 answers
44 views

How to plot right ascension with time of the retrograde motion of mars? [closed]

Measure the right ascension, declination of mars and sun. plot the variation of right ascension with time.
0 votes
0 answers
31 views

Longitude from different vantage points

Given, the mean longitude L of the Earth-Moon barycenter, as seen from the Sun's center. From this, is the mean longitude of the Sun L+180°, or is it 360°-L, as seen from the Earth-Moon barycenter? ...
3 votes
2 answers
276 views

Geographical Point of the Sun

I want to draw a Sun in a 3d scene, so I figured the easiest way would be to calculate the latitude and longitude of the point perpendicular to Sun position and then just move it "up". As a starting ...
2 votes
1 answer
162 views

Calculating Position of Sun in GPS Frame Relative to 0°00'00.0"N 0°00'00.0"E: Determining Vernal Equinox Offset

I am trying to calculate the position of the sun, in GPS coordinates terms, from a given date. As of right now, I know how to accurately calculate the equatorial Right Ascension and Declination, and ...
9 votes
2 answers
1k views

How was the First Point of Aries measured in ancient times?

According to Wikipedia, usage of the Sun's position as the basis for a celestial coordinate system dates back at least to Babylonian times, and the current "First Point of Aries" system ...
1 vote
1 answer
220 views

Angular distance calculation in Equatorial and Galactic coordinates

I calculated the angular distance between the Galactic Center and Andromeda in Equatorial and Galactic coordinates and observed that the distance is calculated differently depending on the coordinate ...
4 votes
3 answers
332 views

Why don't we use the Voyager/Pioneer etc space probes to measure stellar parallax?

With Earth based telescopes you get a max distance of 2 au between measurements. While we have multiple space probes well over 100 AU away from us, that's a 2 OoM difference.
2 votes
0 answers
89 views

Find position of orbiting body along orbit efficiently

I'm coding a game, and a fundamental mechanic is celestial bodies moving in elliptical, Keplerian orbits around a single gravitational point in a planetary system (not our solar system). The bodies ...
2 votes
1 answer
145 views

Discrepancies in the equations for converting Horizontal coordinates to Equatorial coordinates

Hello I am trying to convert my Horizontal coordinate system dataset into Equatorial coordinate system. However, it seems that all the information from the internet is different. I will use symbols as ...
12 votes
2 answers
2k views

What positional accuracy (ie, arc seconds) is necessary to view Saturn, Uranus, beyond?

My son and I are building an azimuth/elevation controller to point the telescope at planets using servos and a microcontroller. Assuming that our device is calibrated (that is, we tell our device to ...
5 votes
3 answers
3k views

How can I convert my sky coordinate system (RA, Dec) into galactic coordinate system (l, b)?

I am an astro-particle physics PhD student trying to look for a simulation. I was looking for a conversion formula for sky (or celestial or Equatorial) coordinate system using Right Ascension and ...
1 vote
1 answer
113 views

Calculate declination of Galactic N. Pole given obliquity of Ecliptic, rt. ascension of Galactic N. Pole, & angle between Galactic & Ecliptic Pole

I’m trying to back-calculate a value in astronomy using spherical trigonometry. (This is not a homework problem, I'm retired). Using the following formula, where δ = the Declination of the Galactic ...
4 votes
1 answer
232 views

CSPICE vs JPL Horizons discrepancy in AZ/EL

This CSPICE spkcpo_c routine example outputs: ...
3 votes
1 answer
90 views

How can we make sure that it is physically the same source when we do cross matching?

When we go through multiple astronomy catalogs, we often do Cross-matching. We compare the position parameters $(\alpha, \delta)$ of different sources (galaxies, quasars, etc.) in the equatorial ...
3 votes
1 answer
393 views

How to calculate Local siderial time that is consistent with Stellarium

I have a number of date and times when I am interested in the location of my zenith and what objects might co-locate with these positions. The easiest way to do this was to put my latitude and ...
1 vote
0 answers
24 views

calculation of observers latitude, first vertical

In an exercise, I have to calculate the observers latitude. Given are the altitude a of the star at the passage through the first vertical (Azimuth = 90°/-90°), the sideral hour, the right ascension, ...
2 votes
0 answers
93 views

How to know if the star is on the west or east side calculating hour angle and sideral hour

I have an exercise where I need to calculate the hour angle (H), declination (δ), latitude (φ) and the sideral hour at the moment the star passes through the 18 hour circle. Given are the right ...
2 votes
0 answers
56 views

You find yourself on a random planet in the milky way 2000 years in the future. Can you figure out where/when you are? [duplicate]

Imagine you're on a planet in a distant part of the milky way, 2000 years from now. You don't know where you are or how much time has passed. You have access to modern astronomical data and technology ...
42 votes
2 answers
7k views

Where can I find the positions of the planets, stars, moons, artificial satellites, etc. and visualize them?

What resources are available to find the positions of planets, stars, moons, artificial satellites, asteroids, and other heavenly bodies?
2 votes
2 answers
1k views

Jean Meeus - calculating moon rise, transit and set times - help isolating error in logic

I've been working my way through the Jean Meeus Astronomical Algorithms book for a few weeks, to implement some of the key algorithms for a personal project. This task has been a little more complex ...
3 votes
0 answers
97 views

Does Earth's movement around the sun contribute to the angular distance between the moon and planets?

This merged image shows the angular distance between the moon and Mars at 10 PM (upper part) and the change in the angular distance at 10 PM the following day (lower part). My understanding is that ...
2 votes
3 answers
2k views

Why do stars seem not to move relative to each other?

Whilst I understand why stars move across our skies,(Earth rotation) I struggle to understand why they do not appear to move relative to each other. To explain: Take the best know constellation the ...
17 votes
2 answers
3k views

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 ...
2 votes
0 answers
71 views

Mathematical condition of Stationary Planet

In an old astronomy book I find the following definitions: $\vec r_1$ Position vector of Earth (heliocentric) $\vec r_2$ Position vector of a Planet (heliocentric) $\vec v_1$ Velocity vector of Earth (...
3 votes
1 answer
114 views

How can I compute the shape and orientation of the crescent for a given time and geographic location using data from the JPL Horizons?

I can compute the following data for the Moon for a given time and geographic location using the JPL Horizons system: Apparent RA & DEC Apparent azimuth & elevation (AZ-EL) Target range & ...
2 votes
1 answer
88 views

Mismatch between the equation of the equinoxes and the correction for a right ascension

I am studying about apparent and mean positions of Sun using quite old books: Astronomical algorithms (1991) and Explanatory supplement to the astronomical almanac (1992). The question is about the ...
5 votes
0 answers
133 views

Positional astronomy - Speed of the sun in sky

Let $\vec{r}$ be the position of the Sun in the sky, from Earth. How could I estimate the magnitude of $d\vec{r}/dt$, to measure the rate of change in time of the arc it traces? I suppose it would be ...
1 vote
0 answers
60 views

moving light with a fading pulse, very bizarre

October 9th southeast of Denver about 6 miles, looking northwest in the sky approximately 10 inches to the left of Alkaid, a bright light, what at first glance might have been a star, began moving ...
2 votes
0 answers
119 views

Tropical or sidereal orbital period when calculation Solar System positions?

I'm reading Practical Astronomy With Your Calculator Or Spreadsheet by Duffett-Smith and Zwart. When calculating the positions (in equatorial coordinates) of the Sun and planets the authors use the ...
1 vote
1 answer
119 views

Meaning of $\cos H$ in the rising and setting formula

In the rising and setting formula$$\cos H=-\frac{\left(\sin\nu+\sin\phi\sin\delta\right)}{\cos\phi\cos\delta},$$ $H$ is hour angle, $\nu$ is vertical shift, $\delta$ is declination and $\phi$ is the ...
5 votes
2 answers
173 views

Measurement unit of coordinate systems in astronomy

Nearly all of the coordinate system I have studied in Positional astronomy use the degree system. This makes me curious about why astronomers prefer degrees over radians as we know that Radian is a ...
1 vote
1 answer
245 views

Translating a zenith position to the nadir

What would be the easiest way to translate a zenith position to a nadir on the celestial sphere? For example I have a position from the azimuthal grid in Stellarium; 3h44m32.64s/+50°50'16.2" ...
1 vote
0 answers
64 views

How to calculate shooting distance for an astronomical object

With a 50mm full frame lens and a 35mm sensor (or film), how far should I be to shoot Earth with earth’s edge just inside top and bottom edge of the resulting image? I don't want any cropping. Earth ...
10 votes
2 answers
2k views

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 ...
3 votes
1 answer
152 views

How can you derive equations between the Horizontal and Equatorial system if the two systems have different centers?

This might be a dumb question. If the center of the celestial sphere is the center of the earth, and the center of the horizontal system is the observer, why do people use this diagram to derive ...
2 votes
0 answers
40 views

LSR and peculiar motion corrections to RAVE survey velocities

I am working towards getting a bunch of halo stars in our galaxy for my thesis. I am doing this with the help of a Toomre diagram and I am stuck. I am using the RAVE survey in conjunction with Gaia as ...