# Tag Info

### How (the heck) does Astrometry.net work?

I've been trying to figure out the technical details of astrometry.net for quite some time. As others already pointed out, the main input to the whole process is a list of stars. I will not go into ...
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### How did ancient astronomers know to order the planets from the closest to the farthest from the Sun?

According to the Cambridge Concise History of Astronomy (p 33 of my edition), essentially the Greeks took the (not unreasonable) view that the planets that moved more slowly were further away and were ...
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### Parallax due to the motion of the Sun

You are referring to what is known as proper motion, which is the apparent motion on the sky due to the relative motion of the Solar System and the stars. To measure an accurate parallax you have to ...
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### What will it finally take to accurately measure the distance to Betelgeuse?

The problem is that the apparent diameter of Betelgeuse is about 50 mas (milliarcsecond --- 1 mas is about 5 nano-radians) while its parallax is about 5 mas and its shape and surface brightness are ...
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### How (the heck) does Astrometry.net work?

Finding an astrometric solution from an image with Astrometry.net is usually called plate solving. As mentioned in the comments, it is based on pattern matching, using a large set of databases that ...
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### Why don't we use the Voyager/Pioneer etc space probes to measure stellar parallax?

The basic reason is that Voyager and friends have cameras optimised for imaging planets, not for doing very precise astronometry. So why not send a telescope, like that on Gaia, into interplanetary ...
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### What is the mean separation between a star and its closest neighbor in the Milky Way halo?

I did some rough calculations, and 100 light-years doesn't seem to be a bad guess. If we assume that the average mass of a halo star is $\sim0.3M_{\odot}$, as would be expected for a typical IMF, and ...
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### What exactly is the orbital period value of Sirius binary star system?

Bond et al. (2017) measure the orbital period of the Sirius system to be $50.1284 \pm 0.0043$ years. I believe this is the most precise and accurate value (I cannot find any more recent papers, with ...
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### Measuring misalignment between two positions on sky

Assuming you mean the angle between the meridian line through A and the great circle that goes through points A and B, then it goes something like this. Define vectors from the origin to A and B ...
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### Where might astrometric observations of the Roadster spacecraft be catalogued?

The Minor Planet Center inconspicuously puts this under "Distant Artificial Satellites Observation." DASO Circulars 561-566 have observations of the roadster 2018-017A. It's also on the short list of ...
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### What will it finally take to accurately measure the distance to Betelgeuse?

So the Hipparchos parallax of Betelgeuse doesn't seem accurate enough? If only someone would launch an even more advanced astrometric satellite than Hipparchos. Actually the ESA has launched an ...
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### Are we less safe now that Arecibo is no more?

The linked article is copied from a university press release. Arecibo's article is more matter-of-fact but naturally also emphasizes the value of their own work. NASA Goldstone can do planetary radar ...
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### 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?

Let me see if I can at least partially answer some of your (many!) questions, How would, or how is commonly, earthshine best stated (absolute magnitude, overall watts from the lunar surface, or?). (...
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### How did they measure the distance to the center of our galaxy to 0.3% accuracy?

The orbit of star S2 is completely determined by the astrometric observations. i.e. One has the orbital period, the angular scale of the orbit and the inclination of the orbital plane. With this ...
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### How was precision astrometry done before digital imaging?

Prior to digital imaging then photographic plate negatives were analysed with scanning microdensitometers to produce astrometric catalogues. Many of these catalogues are still in use today, they are ...
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### How could one use the transit of Mercury to try and find the Astronomical Unit?

Thanks to the Mercury transit, you can measure the parallax from the Earth. That happens due to TRACE , which tracks the transit of Mercury along the polar diameter of the Earth. During that tracking,...

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

Spacecraft such as Pioneer, Voyager or New Horizons are designed to study planets, and their cameras and telescopes to obtain good images of them, not to perform precision astrometry. Anyway, as a &...
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### What is the explanation for discrepancies in RA values at Santa Maria Degli Angeli e dei Martiri in Rome

Here's a little script I wrote that uses Astropy to calculate the right ascension of these stars in 1703 AD, accounting for both proper motion and precession of the equinoxes. It's based on an earlier ...
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### Calculating the distance of objects using the Gaia DR1

The early 1990s Hipparcos mission yielded parallaxes for 118000 stars (Hipparcos catalog) and positions without parallax for another 2.4 million (Tycho-2 catalog). The Tycho-Gaia astrometric solution (...
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### What's the difference between GSR and Grf

Short Answer They're the same. Long Answer Local Standard of Rest (LSR) Let me start out by defining the dynamical Local Standard of Rest (dynamical LSR or also LSRD). This definition is taken ...
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### Measuring misalignment between two positions on sky

The position angle P of a body ($\alpha_1, \delta_1$) with respect to another body ($\alpha_2, \delta_2$) can be calculated from tan(P)={sin(\Delta\alpha)\over cos(\delta_2)tan(\delta_1)-sin(\...
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### Open access table of visible stars with magnitude, coordinates, and possibly color?

The Hipparcos catalogue by van Leeuwen (2007) contains all the information you require, plus estimates of distance from parallax. It is open and free to use for scientific purposes. http://vizier.u-...
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### How can GAIA's accuracy for position, parallax, and annual proper motion for a given star be the same number?

Parallax and proper motion are determined from a series of position measurements taken over the course of (for Gaia DR2) 22 months. A "5-parameter" astrometric model is fitted to these position ...
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### Measurement uncertainty of the computed center of gravity from pixel image

Every pixel value $S_i$ on the detector at $\vec x_i$ has some error $N_i$: CCDs for example have a background noise $N_\text{bkg}$ from read-out electronics, thermal noise, and sky background, plus a ...
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### At what latitude could Orion's belt and Aldebaran be ~10 degrees above the horizon at the same time?

This sky configuration is consistent with 40 degrees N, at about 6.30am (local time) at the end of November. If you went to mid November, this would look the same at about 7.30am (ie 1 hour per half ...
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### What measurements helped us in understanding 'Oumuamua's location and speed?

Like most asteroids and comets, 1I/'Oumuamua's trajectory was determined entirely by measuring its position in optical images over several days. The earliest data came from automated, ground-based ...
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### How to actually obtain the parallax angles that are used to determine distances of stars?

In actual fact its a bit more complicated, because the Earth's orbit is not perfectly circular, The star does not lie exactly on the Earth's orbital plane, so the observation is not a line of ...
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### Measurement uncertainty of the computed center of gravity from pixel image

You might take a look at this 2006 paper by Thomas et al. on centroiding algorithms for astronomical adaptive-optics (AO) systems, which includes a detailed discussion of error estimates for the ...
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### Why does Sirius proper motion "wiggle" with a 1 year period, when Sirius A and Bs binary orbit is a 50 year period?

Your vantage point changes throughout the year as the Earth moves around the solar-system barycentre (close to the centre of the Sun). That means the positions of nearby stars "wiggle" with respect to ...
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Assuming that each (square) pixel has the same angular scale (not a given if the field of view is large) of $\theta$ degrees/pixel. Then the declination (in degrees): \$\delta \simeq \delta_0 + \theta ...