# 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 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 & ...
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### 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 ...
790 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 ...
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1 vote
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### 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 ...
1k 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 ...
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### 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 ...
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### 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 ...
186 views

### 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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### Airy's Water Filled Telescope: help required to fully understand the calibration procedure followed

I am reading/studying the instrument description of Airy's Water Filled Telescope (Airy, G. B., "History and Description of the Water Telescope of the Royal Observatory, Greenwich", ...
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### Compute Distance Between Stars

If I have the following information about star A and Star B, how can I compute the distance between A and B? Distance from Sol for Star A Right Ascension/Declination of Star A Parallax/Absolute ...
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### Small Angle Approximation Discrepancy in Horizon Dip Angle

Let's get back to the classic astronomical problem of calculating horizontal dip angle using small angle approximations. Let, our observer is a point object situated at an height $h$ on the earth ...
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### How to determine sidereal time just looking at the picture?

I have this picture: I need to determine what sidereal time it is (in hours), or how long has it been since the upper culmination of the point of the vernal equinox. How can I do that?
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### Knowing location, LST, the amount & direction of displacement of an unknown star in a 2d image over time, can you calculate the coords of this star?

Basically I have camera, my location, my local sidereal time. I want to point the camera in one direction and shoot a timelapse of a certain star exactly in the middle of the camera view. So I know ...
319 views

### Rising and setting time of planets in relation to the sun

If Venus set about 2 hours after the Sun,then it should be about 30degrees to the east of Sun. Is there any mathematical formula to get these thing or it's just a pattern? Source :https://olympiads....
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### Tips of Crescent

Can it be shown mathematically that the line connecting tips of crescent is parallel to the North-South line and the line gives latitude of the plane ( When the Moon is sufficiently close to the ...
1 vote
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### Angle of Declination and Culminations

Is it true that angle subtended by the Zenith Distance at Upper and Lower culminations is equal to the declination of the star ?If It's true then why? Is the motion of Earth make the solid angle of ...
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### What is the connection between declination and latitude?

The celestial equator is a projection of the terrestrial equator on the celestial sphere. So doesn't that mean the geographical latitude and the declination will be "exactly" equal, as they ...
186 views

### Mathematics for Conversion between ICRS and AltAz

I am writing some software, for my own learning experience, to convert astronomical coordinates to different frames of reference. An example of this would be conversion between ICRS which a lot of ...
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### predicting angular seperation between planetary objects and earth satellites

I am struggling to calculate the angular separation between a satellite in earth orbit and a planetary object using the Skyfield library. It's equivalent to calculating the angular separation between ...
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### Altitude of Polaris is latitude

How to prove geometrically that the altitude of Polaris is equal to the latitude? I have seen in handouts that the altitude of the pole star gives the latitude of that place.
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### Rising time of a planet or a star and its correlation with the angle subtended at the Earth

While preparing for Astronomy Olympiad I came across many problems in which we had to calculate at what time a planet or a star would rise at the Earth, and to solve this many solutions found the ...
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### Calculating the position of stars at a given point in time

Is there any particular methods that help us calculate the position of a star at a certain point in time? This question is what I am trying to achieve, but for me, I do not want to use the data ...
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### What is the meaning of integrated emission maps for these chemical species?

In the Astrobites article Spectral Line Survey Reveals New Molecules in Two Protoplanetary Disks integrated emission maps depict the spatial distribution of the flux received from each "line.&...
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### Where can I find planetary rotational axis direction?

I can find axial tilt of planets easily, but that doesn't specify the direction of that tilt, i.e. planet's rotation axis may be anywhere in circle defined on a sphere by axial tilt value. And I can't ...
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### Why is Earth at centre of Celestial Sphere?

Why do we consider Earth to be at the centre for Celestial Sphere ? If so, then how will Sun revolve around Earth ? Doesn't it mean its like the geocentric model ?
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### Straightforward derivation of the sunrise equation

I'm looking for a reasonably straightforward derivation of the sunrise equation$$\cos\omega=-\tan\phi\tan\delta$$as referenced in the Wikipedia article here. I've dipped into Astronomical Algorithms ...
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### consider you are stranded in deep space, and an alien agreed to drop you back home, how would you explain the location of earth in the galaxy? [closed]

remember they have no concept of what humans have named stars and clusters
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### Question on positional astronomy

I was preparing for an Astronomy Olympiad which has this question: On the evening of an autumnal equinox day Siddhant noticed that Mars was exactly along the north-south meridian in his sky at the ...
396 views

### Calculating the position of the Sun

I'm reading the book "Practical Astronomy with your Calculator or Spreadsheet 4th Edition" implementing its formulas with C++. Now, I'm implementing formula 46 "Calculating the position ...
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### How precise are Saemundsson/Bennett refraction formulas?

I was trying to reproduce the predictions of the Bennett formula* by physically-based calculations with a model based on a real-life refractivity profile. My calculations based on Table V from ref. 1 ...
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### Best Books on Spherical Astronomy

I am preparing for an astronomy olympiad and a good amount of questions in the olympiad come from Spherical astronomy, some of the books ive tried reading were too hard to understand as this topic ...
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### How can you determine the date using nothing but a star chart from the day and the time?

I'm doing a science olympiad event called Reach for the Stars, and one question that I've encountered provides a star chart and time of day, and you must figure out what month of the year it is from. ...
1 vote
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### Finding Observer Latitude/Longitude - In Simple Terms

I was wondering if someone could explain to me in simple terms how to determine the observer latitude/longitude at a given location. For instance, if an observer is positioned at Cleveland, Ohio, how ...
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### A comet orbits the Sun in the same plane as Earth's orbit

If the Earth around the Sun follows a practically circular orbit of radius equal to one astronomical unit, determine its orbital speed in astronomical units per year. If the Earth around the Sun ...
63 views

### Secular parallax calculation in Binney and Merrifield

I am now reading Galactic Astronomy (1998) by Binney and Merrifield. In this book at p. 44 there is this formula for calculating proper motion vector: \boldsymbol\mu_i = \frac{(\mathbf u_i \times\...
1 vote
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### 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" ...
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### Constellation a celestial body is in

Given its right ascension and declination, find out the constellation a celestial body is in. Does anyone know of an algorithm (C#, js) for that? It would read the constellation boundaries (e.g. ...
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### About coordinate systems and angle differences

If I want to measure the beam-width of a radio telescope sweeping through a ~ puntual object (the Sun) with a ~ constant flux output when measured: The value measured sweeping in one frame of ...
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### Stellar Aberration and Lorentz Transformation

The aberration of starlight is claimed to result from the Lorentz transformation of Special Relativity, but I am having problems to understand this. Assume we have two reference frames $x,y,z$ (the ...
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### How to manually locate a certain declination in the sky for a given location? (no sextant available)

Consider this example. I am standing in a flat surface, at latitude X, looking to the equator, in a random day of the year. (For simplicity, let's assume X > 23.5º) I want to find the position of ...
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### How to I calcuate observed position of the Sun and Moon using the SOFA C library?

There are a number of answers on here that skirt the topic, but no concrete answers. I currently have an implementation based on "Astronomical Algorithms by Jean Meeus" for stars, moon, and ...
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### Confused about the significance of sidereal time

My understanding of sidereal time is that the position of a star will be in the same place 4 minutes earlier the next day. So a 20 days later the star will be 1hr 20 mins earlier. So if the star is in ...
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### What are the correct positions or alignment of planets in relation to each other and the Sun, is it vertical center, baseline or completely random

From a lot a illustrations and videos, the planets seem to align in vertical center to each other like this: Is this really true? I like to imagine that the planets are actually in weird alignment to ...
161 views

### The Jupiter Effect Book

This evening I have been thinking about the anxiety caused in 1982 by predictions in John Gribbin and Stephen Plagemann 1974 book The Jupiter Effect. I was a teenager in 1982. The planets aligning ...
126 views

### What measurements helped us in understanding 'Oumuamua's location and speed?

What measurements helped scientists understand 'Oumuamua's location and speed? I wanted to know how it was achieved and with what accuracy if possible.
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### Right ascension, Equinox, Positions Help

I am trying to answer the following question which I slightly understand but need a clearer explanation: What are the times of year to observe an object at X hours of RA at visual wavelength? I am ...
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### What are the Mathematical Algorithms/Formulas behind hindu Sunrise and Sunset Calculations?

There are differences between Astronomical Sunrise/Sunset calculation and Hindu calendar Sunrise/Sunset calculations. The astronomical algorithms to calculate the sunrise/sunset can be found over the ...
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### Where can I get J2000 epoch initial mean longitudes for all planets and celestial bodies?

I'm following this paper to calculate geocentric coordinates. The authors have provided the initial mean longitude values for the planets (provided in attached table) for 01.01.2000 @ 00.00 UT but I'...
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### Converting rectangular to polar coordinates

I'm following the steps in this paper on how to calculate geocentric longitude. In section 3.4.2 it says the Geocentric rectangular co-ordinates of Mercury then are: ...
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