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Questions tagged [astrometry]

Questions about the measuring the position of stars, and the techniques used to do this.

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What will it finally take to accurately measure the distance to Betelgeuse?

Comments below What is the maximum distance measurable with parallax? discuss challenges associated with parallax measurements of Betelgeuse and link to Wikipedia’s Betelgeuse; Distance measurements ...
uhoh's user avatar
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13 votes
2 answers
2k views

How does angular resolution of a telescope translate to its parallax precision?

We can often read in the scientific and also more casual reader literature and articles about the angular resolution of various telescopes and other optical equipment, be it ground based or onboard ...
TildalWave's user avatar
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12 votes
2 answers
3k views

How (the heck) does Astrometry.net work?

What magic is this? This answer to my question Astronomy detective question: what part of the sky are these photos of? What are a few of the stars? nails it, and the output (linked there) contains ...
uhoh's user avatar
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10 votes
1 answer
443 views

If the Wow! signal was not directed at us, then where?

We probably all know the Wow!-Signal, happened on August 15, 1977. Assume that it wasn't meant for us. What was it pointing at then? The signal at that time came from RA= 19h25m31s ± 10s or 19h28m22s ...
draks ...'s user avatar
  • 385
9 votes
1 answer
3k views

How did ancient astronomers know to order the planets from the closest to the farthest from the Sun?

If we assume a geocentric model of the universe (like ancient astronomers did) how could we ever find out (again, like ancient astronomers did) the right distances of the planets? For example, how did ...
Carlos Vázquez Monzón's user avatar
8 votes
1 answer
612 views

Are we less safe now that Arecibo is no more?

update 3: uhoh! Source: Reddit update 2: Sadly, parts of the dish have collapsed, several cables have snapped, and the Arecibo radio telescope is now decommissioned and demolition has been ...
uhoh's user avatar
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8 votes
1 answer
149 views

Anecdote of an astronomer looking for correlations in antipodal pairings of stars; what am I remembering?

The question Are the stars distributed in uniform distribution, on the celestial dome, with respect to brightness? brings to mind a different study that I vaguely remember hearing about a while ago, ...
uhoh's user avatar
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6 votes
2 answers
1k views

Parallax due to the motion of the Sun

Generally, the parallax is contributed to by the motion of the Earth around the Sun. However, at the same time, the Sun is moving too. It carries the Earth with it. Presumably, the shift of the Sun is ...
wdlang's user avatar
  • 259
6 votes
1 answer
622 views

Where might astrometric observations of the Roadster spacecraft be catalogued?

above: (see small red lines near center of image) Tesla Roadster. 18 Feb. 2018, 11:28 UT. The image above comes from the average of ten, 300-seconds exposures, remotely taken with the 16”-f/3.75 ...
uhoh's user avatar
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6 votes
1 answer
139 views

Has earthshine in fact increased since lights (say 1900); that is to say can we measure or has there been a measurable increase? What are the numbers?

Three questions, How would, or how is commonly, earthshine best stated (absolute magnitude, overall watts from the lunar surface, or?). (Further, since the moon is irregularly shiny is it measured ...
Fattie's user avatar
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6 votes
1 answer
197 views

Determination of High-Accuracy Distances of Terrestrial Planets from the Sun

I understand how in olden days the Sun-Planet distances were estimated using:- (i) measures of planet orbital periods ($T$) from analysis of observations over several centuries; (ii) Kepler's 3rd Law $...
steveOw's user avatar
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6 votes
0 answers
135 views

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", ...
JKrsl's user avatar
  • 121
5 votes
3 answers
341 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.
blademan9999's user avatar
5 votes
4 answers
871 views

Measurement uncertainty of the computed center of gravity from pixel image

Establishing the position of celestial bodies (stars, and other objects) is based on imaging technology. Depending on the resolution of the imaging sensor used, an object is identified by a group of ...
Match Maker EE's user avatar
5 votes
1 answer
240 views

How could one use the transit of Mercury to try and find the Astronomical Unit?

I am trying to use the transit of Mercury to estimate the distance between the Earth and the Sun, or 1 AU. I know that I need to observe mercury from two antipodes, but I do not understand any of ...
Khanrad's user avatar
  • 63
5 votes
1 answer
221 views

Gaia: What is the difference between CCDs used for astrometry, photometry, and spectroscopy?

My knowledge of CCDs is that these are sensors which collect photoelectrons. That's about it. What is a difference between CCDs used for astrometry, spectroscopy, and photometry? As an example, each ...
ShanZhengYang's user avatar
5 votes
0 answers
134 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 ...
nuwe's user avatar
  • 771
4 votes
3 answers
942 views

Measuring misalignment between two positions on sky

Let's say I have the (RA,Dec) for two positions on the sky. I want to measure the position angle between these two positions. In other words, if I had the Cartesian coordinate vectors for these two ...
quantumflash's user avatar
4 votes
2 answers
293 views

What is the mean separation between a star and its closest neighbor in the Milky Way halo?

My question is inspired by this previous question, Are there any stars that orbit perpendicular to the Milky Way's galactic plane?. In the great answer posted by @ConnorGarcia, one commenter said he ...
ksousa's user avatar
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4 votes
2 answers
548 views

How to actually obtain the parallax angles that are used to determine distances of stars?

I’m trying to understand how to measure the distance to nearby stars using the parallax method. My question is: how do you get the value of the p angle?
codependent's user avatar
4 votes
1 answer
551 views

How did they measure the distance to the center of our galaxy to 0.3% accuracy?

The abstract of A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty (Open Access) says: We present a 0.16% precise and 0.27% accurate determination of R₀, the ...
uhoh's user avatar
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4 votes
2 answers
1k views

Plotting Equatorial coordinates to X,Y plane (simulating telescope/camera view)

I would like to simulate a telescope camera view, projecting (RA, Dec) coordinate sets to a 2D plane. So effectively the need is to convert Equatorial coordinates to (x, y) coordinates, when we can ...
AstroDuck's user avatar
4 votes
1 answer
1k views

Practical ways of measuring the sun-earth-moon angle?

I learned as a child (and a common tale), that Aristarchus gave a method of measuring the sun-earth-moon angle to estimate the ratio of the earth-moon distance to the earth-sun distance. Indeed, ...
bonsoon's user avatar
  • 143
4 votes
1 answer
168 views

How can I replicate AstroImageJ's pixel to RA/Dec algorithm in my own code?

EDIT: solved, thanks to Eric Jensen's suggestion (in a comment on his answer) that I include the correction to my right ascension value as a function of my declination. All other comments were helpful ...
Conor W's user avatar
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4 votes
1 answer
143 views

How often are radio telescopes used to measure parallax? When was this first done?

The abstract of A magnetar parallax (also in MNRAS) says: TE J1810-197 (J1810) was the first magnetar identified to emit radio pulses, and has been extensively studied during a radio-bright phase in ...
uhoh's user avatar
  • 30.7k
4 votes
0 answers
126 views

Is there a website or app that can add declination and RA grid to an existing photo?

For a few years now, I've been shooting photos of the night sky with just a tripod and my Nikon DSLR. After extensive post-processing to mitigate light pollution, I usually submit the final results ...
user382459's user avatar
3 votes
3 answers
183 views

Reverse polynomial model for reverse astrometry

I have a polynomial model that can transform pixel X,Y coordinates to standard $\xi, \eta$. This model looks like this: $\xi/\eta = a_1 + a_2x + a_3y + a_4x^2 + a_5xy + a_6y^2 + a_7x^3 + a_8x^2y+a_9xy^...
jlipinski's user avatar
  • 297
3 votes
3 answers
1k views

Calculating the distance of objects using the Gaia DR1

The Gaia DR1 was released middle of September with ~1.1 billion objects. The schema contains right ascension, declination and parallax, which could be used to calculate Cartesian co-ordinates (xyz). ...
soupy_norman's user avatar
3 votes
1 answer
143 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.
Mahen's user avatar
  • 73
3 votes
1 answer
254 views

Radial velocity

Is there a way to calculate the radial velocity without wavelength? I can only find two formulas so far. One of them needs wavelengths and the other one needs the velocity of the star, but I have none ...
C H's user avatar
  • 71
3 votes
1 answer
406 views

Why does the altitude-azimuth mount have a blind spot near zenith?

I understand the subject mathematically. Azimuth of the celestrial object under track changes a finite 180 degrees instead of a infinitesimal amount as the body passes zenith. This would require ...
Cici Xu's user avatar
  • 31
3 votes
1 answer
435 views

Why does Sirius proper motion "wiggle" with a 1 year period, when Sirius A and Bs binary orbit is a 50 year period?

Some stars, if observed repeatedly over time, show a perturbation or "wobble" in their proper motion. If this is a periodic occurrence we can infer that the perturbation occurs due to the ...
memex's user avatar
  • 101
3 votes
1 answer
903 views

Transform pixel coordinates (in FITS file) to equatorial

Say I have a FITS file with an observed field of stars, each one with x,y coordinates given in pixels. Assume the image is aligned so that the x axis is the right ascension, and the y axis the ...
Gabriel's user avatar
  • 822
3 votes
1 answer
269 views

I can't find Rigel on Tycho 2 catalogue

I'm trying to develop a planetarium using Tycho 2 catalogue. To read it I use the WCSTools. But I have a problem. On Wikipedia I have found that Rigel star has RA: 05h 14min 32,3s and Dec: -08º 12’ ...
VansFannel's user avatar
3 votes
0 answers
48 views

Tools and algorithms for angles-only orbit estimation

I've seen discussion threads where amateur astronomers estimate a satellite's orbital parameters from angles-only telescope observation. Are there any community tools or algorithms to help with this ...
Carl F.'s user avatar
  • 131
2 votes
2 answers
1k views

What exactly is the orbital period value of Sirius binary star system?

My Textbook An Introduction to Modern Astrophysics 2nd edition (2017), page 557 mentions that the modern value is 49.9 years. But Wikipedia mentions 50.1284±0.0043 years. Which is correct? Or, has the ...
James Smith's user avatar
2 votes
1 answer
143 views

At what latitude could Orion's belt and Aldebaran be ~10 degrees above the horizon at the same time?

In his appearance on television contained in the video The Mystery That Keeps Neil deGrasse Tyson Up At Night, in the backdrop behind astronomer and educator Neil deGrasse Tyson and the talk-show host,...
uhoh's user avatar
  • 30.7k
2 votes
1 answer
664 views

Celestial coordinates of every pixel in the image

I am solving starfield using Astrometry.net, an amazing piece of software I must say. After solving the starfield, Astrometry gives celestial coordinates of centroids of every star in the image, as ...
Falco Peregrinus's user avatar
2 votes
1 answer
166 views

How was the Messenger spacecraft used to measure Mercury's orbital precession to such accuracy? Could this have been done using radar?

This comment under an answer to Path of Mercury and general relativity mentions that the Messenger spacecraft was used to measure the precession of Mercury's orbit to such accuracy that the tiny ...
uhoh's user avatar
  • 30.7k
2 votes
1 answer
78 views

Are astronomers continuously monitoring exoplanetary systems?

I'm reading many of the Wikipedia pages about exoplanets and the different methods they are using to detect them. But I wonder, it seems that the emphasis is on detecting and finding new exoplanets, ...
Kevin Holmes's user avatar
2 votes
1 answer
2k views

how to plot sky map like www.nakedeyeplanet.com with dates tags with planetary motion path

I am curious how to plot planetary motion star chart with date tags like on www.nakedeyeplanets.com by Martin J. Powell. Is there any way to generate images similar like that. What software should ...
M10 Radalpha's user avatar
2 votes
1 answer
109 views

Can angular information be known more precisely than the diffraction limit?

The diffraction limit deals with the ability to determine if two things are separate. I am interested in the ability to find the the centroid of a single object. Imagine a star with no near ...
Darrin Taylor's user avatar
2 votes
2 answers
133 views

Determing timestamp of a photo of the stars using a plate solver

This question from 2016 seems to suggest that you need a powerful telescope to get a photo of the stars in such a way that you can use their proper motion to get a timestamp. If this is possible, how ...
requiemman's user avatar
2 votes
1 answer
176 views

How to interpret error of absolute magnitude of a star given data from wikipedia?

This Wikipedia page contains information about the star Spica in the rightmost table. A small snapshot of this table is below. In this table, the absolute magnitude is given as ...
user avatar
2 votes
0 answers
54 views

How to reduce Hipparcos catalog star coordinates to another Epoch?

Hipparcos catalog lists star equatorial coordinates (RA,Decl). The coordinates are 1991.25 positions on ICRS reference frame (epoch J2000). According to Astronomical Algorithms (J. Meeus) the ...
Omar Reis's user avatar
2 votes
0 answers
343 views

Right Ascension, Declination from pixel coordinates

I have an Excel spreadsheet that will take certain keywords from a FITS image header. Image centre RA, Dec, rotation, x centre, y centre, x scale, y scale. I enter the pixel coordinates of an object ...
George Carey's user avatar
2 votes
0 answers
145 views

How was the astrometry of the famous 1919 solar eclipse confirmation of General Relativity calibrated and verified?

The new New York Times article The Eclipse That Made Einstein Famous describes several events and discussions on the hundred year anniversary of the the 1919 solar eclipse and verification of the ...
uhoh's user avatar
  • 30.7k
2 votes
0 answers
149 views

How to calculate the various properties of a star?

I am wanting to plot a velocity profile (a plot of velocity against the star's distance from the galactic core) from the Gaia DR2, but so far my efforts have been disappointing. The information I ...
Beta Decay's user avatar
2 votes
0 answers
484 views

How do you find the positions of celestial objects in the sky?

I want to write a program which, when given a date, a time, latitude and longitude, returns a list of the planets, Messier objects and NCC catalogue objects what are visible in the sky at that time ...
Beta Decay's user avatar
1 vote
3 answers
540 views

the initial astrometry calibration for a telescope

Generally a telescope can memorize how to point to a target with a particular RA&Dec. There is no need to calibrate every day. How can it happen? How to let the telescope(non-amateur) know the ...
questionhang's user avatar
  • 3,137