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24 votes
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Could we parallax measure stars just based on the Earth's size?

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; ...
pela's user avatar
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14 votes

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 ...
ProfRob's user avatar
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12 votes
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What is the furthest star for which we can measure the diameter?

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 ...
ProfRob's user avatar
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12 votes
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Can New Horizons be used to measure the distance to Betelgeuse (despite its fickle photocenter)?

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 ...
ProfRob's user avatar
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11 votes

Does stellar parallax only occur parallel to the ecliptic?

Parallax motion is seen on the plane of the Earth's orbit projected onto the sky towards the star in question and is not restricted to motion parallel to the ecliptic plane. The parallax motion of a ...
ProfRob's user avatar
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9 votes

What is the proper interpretation of a negative parallax?

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 ...
BowlOfRed's user avatar
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9 votes

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 ...
James K's user avatar
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8 votes
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How does a Solar Transit give more accurate determination of Earth-to-Sun distance?

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 ...
terry-s's user avatar
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8 votes
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How are SIMBAD's parallaxes calculated?

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 $3.78\pm 0.34$ ...
ProfRob's user avatar
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8 votes

Does stellar parallax only occur parallel to the ecliptic?

Does stellar parallax only occur parallel to the ecliptic ...? No. For example, Polaris has an observable parallax, and that's because Polaris is a somewhat nearby star (about 99 parsecs or 323 light ...
David Hammen's user avatar
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7 votes

What's the farthest object as determined only by parallax?

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 ...
AspicioAstra's user avatar
7 votes
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Why is WISE J0521+1025's distance so uncertain?

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 ...
ProfRob's user avatar
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6 votes

Gaia Parallax data

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. ...
Mike G's user avatar
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6 votes

Using parallax to find distance to the moon

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 ...
James K's user avatar
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6 votes

How exactly was Giovanni Cassini able to measure the distance to Mars?

This is my first post on Stack Exchange so bear with me, I'm new to this. I've been looking into the Cassini / Richter measurement for quite some time, and I think I have something to add here. Using ...
John Read's user avatar
6 votes
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vertical shift is equals to half of angular dimension plus parallax plus atmosphere refraction

According to the Astronomical Almanac, the zenith distance for the Moon when rising or setting is $90° 34'+s-\pi$ where 34' is due to refraction, s is the semidiameter, and $\pi$ is the horizontal ...
JohnHoltz's user avatar
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6 votes

What is Mercury and Venus’ apparent retrograde motion?

Welcome! So, the way we define retrograde motion for celestial objects isn't the same way we define it, or even demonstrate it here on Earth, as in, the car next to you appears to be moving backwards ...
ChairmanMeow's user avatar
5 votes

What is the proper interpretation of a negative parallax?

It depends how negative the parallax is and what your "prior" knowledge is of the distance to the star. As another answer suggests, there are some spurious large negative (and positive) ...
ProfRob's user avatar
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5 votes
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What's the farthest object as determined only by parallax?

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 ...
ProfRob's user avatar
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5 votes
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Why no Polaris distance in GAIA DR2?

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 ...
Mike G's user avatar
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5 votes

Stellar parallax

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....
ProfRob's user avatar
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5 votes
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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,...
Carlos Vázquez Monzón's user avatar
5 votes

What is Mercury and Venus’ apparent retrograde motion?

Apparent retrograde motion for the inferior planets occurs when the planet is moving between the Earth and the Sun. Because the inferior planet is moving faster than the Earth around the sun, it ...
notovny's user avatar
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5 votes

Does stellar parallax only occur parallel to the ecliptic?

Perhaps a simpler explanation: Imagine traveling in a straight line down a long road. You will observe parallax of objects in every direction, except objects you're heading directly towards. For all ...
Greg Miller's user avatar
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5 votes

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 &...
Albert's user avatar
  • 2,102
4 votes

What was Bradley's observation concerning abevation of light?

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 ...
Ken G's user avatar
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4 votes

What is the proper interpretation of a negative parallax?

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 ...
nmg's user avatar
  • 41
4 votes

What is the proper interpretation of a negative parallax?

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 ...
Andrew's user avatar
  • 41
4 votes
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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 ...
ProfRob's user avatar
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