# Tag Info

23

In principle, it's not impossible. The Gaia spacecraft, designed primarily for measuring stellar positions, is able to measure parallaxes up to 10 kpc away with 20% uncertainty. Its baseline is 2 AU; $2.3\times10^{4}$ times larger than the diameter of Earth. Thus, placing two Gaias on each side of Earth would be able to measure parallaxes of stars up to a ...

14

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 observe for long enough that you can disentangle the parallax motion, which is of course modulated on a 1 year cycle, from the proper motion of a star, which ...

13

Stellar Parallax Stellar parallax uses differences in perspective to determine the distance from an object. When the earth goes around the sun, our perspective of the star, galaxy etc. changes and so the angle from us to the object changes. Because we know how the earth moves around the sun, we know the distance between the points that we take the ...

12

It's neither the angular diameter or prallax precision that is the limiting factor, but the fact that it is difficult to get the interferometric measurements for faint stars. State-of-the-art angular diameters are measured by infrared interferometry (e.g. with the CHARA array - Gordon et al. 2019). The most precise measurements of angular diameters have an ...

10

The parallax is found from a triangle with the Earth's orbit at its base. The annual parallax here would then be between 1/200 and 1/1000 radians ($\sim$ 1000 to 200 arcseconds) which is indeed enormous and much bigger than the likely proper motion during a year.

10

In principle yes, in practice no. The telescope is good enough, but the CCD camera will saturate, preventing a good positional measurement. Salient facts. The parallax to Betelgeuse as seen between Earth and New Horizons will be about 250 mas. The current distance uncertainty is ~20%, so to better that, the position of Betelegeuse measured by New Horizons ...

8

No, the telescope doesn't measure the parallax. A sextant or any other angle measuring device fit on the telescope does. And, we don't(can't) directly measure the parallax angle. Instead, we just track the position of the star/object throughout the year. A little bit of spherical astronomy math shows us that the path of a star in the celestial sphere ...

8

The problem of determining the solar parallax (and inversely the Sun's distance from Earth) by classical optical astronomy has been in most periods since the 17th century dominated less by principle than by the great difficulties of controlling and reducing the measurement errors of the very small angles involved (the solar parallax is only about 8.794" of ...

8

Trigonometric parallaxes are measured, not calculated. Simbad reports the source of its parallax measurements if you look carefully. For example, Simbad reports Rigel's parallax as $3.78\pm 0.34$ milli-arcsecs, which comes from the revised Hipparcos catalogue of van Leeuwen et al. (2007) (which will be the case for many bright stars that do not feature in ...

7

Did you read this section of the documentation? It suggests there are ways to deal with it, but I have not examined the paper it refers to. • For closely aligned sources (separated by 0.2–0.3 arcsec), which are only occasionally resolved in the Gaia observations, confusion in the observation-to-source matching can lead to spurious parallax values which ...

7

The first data release from GAIA was in 2016 so fairly new. If you want to be certain you should take a look at it because as you say, GAIA will no doubt be the source of the most accurate parallax measurements in the near future. Speaking of accuracy, I believe that this is the essence of your question because it basically depends on how uncertain ...

7

The distance is that reported by Bihain et al. (2013), which is based on a mean relationship between absolute magnitude and spectral type that has a lot of scatter. i.e. in contrast to most (all?) the other objects in that list, there is no reported trigonometric parallax measurement for this very faint T7.5 brown dwarf. In fact, if it turned out to be a ...

6

Gaia data release 1 was announced in September 2016. It has parallaxes for 2 million stars previously observed by the Hipparcos mission, a small fraction of the 1.1 billion positions recorded so far. Future releases will of course have more data and smaller errors. Gaia has ~600 light curves for Cepheids but probably not parallaxes for most of those. ...

6

You compare the position of the moon with that of the background of stars. If you both take a picture of the moon at the same time, and compare the angle that it makes with a distant star the moon will be in a different position, and from this you can calculate the angle. Here is your diagram showing, in slightly simplified way what I mean. In practice ...

5

It depends what parallax uncertainty you are prepared to tolerate. Very Long Baseline Interferometry (VLBI) at long wavelengths currently provides the most precise parallaxes. Parallaxes to bright radio sources measured in this way can have precisions of around 10 microarcseconds (see for example Reid et al. (2014). According to the review by Reid & ...

5

My query for stars north of +75° and brighter than magnitude 4 returns only 6 stars, brightest of which is γ Cephei. Polaris may be too bright for Gaia. Even if it doesn't saturate the detector, a bright star will probably have larger astrometric errors than a star between magnitudes 6 and 12. DR2 coverage of magnitude 5 and brighter stars is better than DR1 ...

5

The probably most advanced system for determination of parallaxes is AGIS as used for Gaia. It's able to go far beyond the angular resolution of the telescopes. Angular resolution is just one parameter. Actually it's just necessary to determine the luminosity centroids of the stars, almost independent of the resolution of the telescopes. That's mainly a ...

5

Parallax measurement in practice is not as is explained above using the popular diagram you have used. The parallax causes the star to prescribe an ellipse in the sky, the semi-major axis of whose is equal to the parallactic angle. The telescopes generally measure the shift in co-ordinates of star(RA and Dec) and then translate the information to that of ...

5

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, the transit of Mercury goes like that: [ Now notice that, if TRACE remained stationary, the transit would be a straight line. So, if you calculate the ...

4

There is obviously the practical problem that we cannot observe from the Sun! But of course there would be a time of year when the Earth was on the projection of the line CD. The cartoon is just that. In practice, measurements are taken at different points in the orbit on multiple occasions over several years. Two measurements would not be enough to ...

4

You are right that what you must use as a referent is different-- for parallax, your referent will be a much more distant object, because it shows little parallax. In aberration, it happens to the whole visual field. But what is shifted is the angle relative to the ground at which the light arrives, so if you set your telescope to see a given star, in ...

4

Although should not use the negative parallaxes, you should not ignore them either. If you are looking at populations of objects, removing those with negative parallaxes will lead to significant bias in your results, as Luri et al. 2018 has shown.

4

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 measurements, consisting of a sky position at some epoch, a parallax and a proper motion in each of the celestial coordinates. The precisions of each of these ...

4

Let me see if I can show why your "inverse-square-law means you can't get light from distant sources" intuition is wrong. For the sake of argument let's assume stars really are point sources, and look at how much light you would receive from an infinitely old, infinitely large universe uniformly filled with point-source "stars", each with luminosity $L$, at ...

4

First, to measure the angle accurately, you need a point of reference at approximately infinite distance. In astronomy, this is in the form of very distant stars; for your earthly example, perhaps it can be an even more distant building almost on the horizon. Next, we look at the angular separation between the object with significant parallax, and the object ...

4

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 movement but an ellipse both the Sun and the Star are moving relative to everything, so the ellipse is not an ellipse but a helix Observational errors from various ...

3

Negative parallaxes can be interpreted as the observer (in this case Gaia satellite) going the "wrong way around the sun" as mentioned in this Jupyter Notebook by Anthony Brown. This notebook is meant to supplement the Luri+ 2018 paper that has been mentioned in other answers and comments here.

3

It depends how negative the parallax is and what your "prior" knowledge is of the distance to the star is. As another answer suggests, there are some spurious large negative (and positive) parallaxes for faint, crowded sources. If possible, these should be removed. If this is not the case, and the parallax is negative, but close to zero within its ...

3

In addition: It's often better to measure the angular distance to reference stars, instead of absolute angles, because those angles can be measured with much higher accuracy.

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