Parallax angle?

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Harshil Canada
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Parallax angle?

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Post by Harshil »


Hello Astronomers,
Today, I read about parallax distance measurement technique that how we can measure the distance of the star using this method as given in the link
https://www2.jpl.nasa.gov/teachers/atta ... allax.html

But there is little-bit confusion...
we need basically two parameters to calculate distance using parallax method (Distance between Earth & Sun, which is 1AU and angle theta)

Case-1 If we have distance between earth & sun and angle theta then we can calculate distance

OR

Case-2 If we have distance between earth & sun and distance of the star from earth then we can calculate angle theta,right!

Now consider case-1 that I want to calculating distance of star sirius from earth, I know the distance between earth & sun, but exactly how to calculate that parallax angle theta ? to calculate distance...
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Re: Parallax angle?

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Post by KathyNS »


You measure the angle. Since the parallax technique only works for nearby stars, you can safely assume that the majority of the stars in the FOV are much farther away. So you measure the position of the star you are analyzing relative to the more distant stars.

Amateur equipment can measure a 1 arcsecond difference in position between images made six months apart. That corresponds to a distance of 1 parsec. Professional equipment, with much larger apertures, can measure smaller angles, and therefore greater distances.
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Re: Parallax angle?

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Post by yobbo89 »


My understanding is that this only works on nearby stars and other methods are needed to calculate distance of stars futher away.



I guess you would use arc seconds with a telescope to get the angle of the star or the distance the star moved from the back ground then convert it to parsec


https://sciencing.com/convert-arcsecond ... 70742.html

https://www.google.com/search?q=parsec+ ... e&ie=UTF-8
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Re: Parallax angle?

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Post by DEnc »


A catch is that if the star is close enough to easily pick up parallax, its proper motion might be evident too.

Here's a plot of nearly two years of movement of Barnard's Star as picked up with my 4" refractor. Its proper motion is approximated by the red arrow. The blue trace is meant to represent its apparent movement across the star field, which consists of parallax + proper motion, described by the sum of elliptical and linear displacements. The data need to be fit to these two component functions to distinguish parallax and proper motion (work in progress). There's a lot of fun math here (that I need to learn); e.g., the eccentricity of the ellipse is determined by the star's altitude on the ecliptic grid, and its parallax.

But back on point, Barnard's Star's parallax = 0.55 arcsec, but its proper motion in right ascension = 0.80 asec/year.
BarnardsStar.png
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Re: Parallax angle?

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Post by TCampbell »


While it is true that you need to know the distance between the Earth & Sun... the angle you're actually using is based on where the Earth is today... vs. where the Earth is 6 months from now (on the other side of solar system). This means the angle turns out to be 2x the Earth/Sun distance (2AU -- not 1AU).

If you can increase the distance from your two observing sites, then you increase the angle an that means you can get a more accurate measurement *and* you can also measure objects that are farther away. So... NASA is doing some experiments with the New Horizons space craft (the probe that flew past Pluto) ... Pluto has a mean distance from the Sun of about 40 AU ... but New Horizons is much farther... as of today (May 29, 2020) the spacecraft is a little over than 47 AU from the Sun (over 46 AU from Earth). That greatly increases our ability to do parallax measurements.

In addition to knowing the distance between your two observing locations... you also need some known distant point that is presumed to be not moving. Everything is moving... but the farther away the object, the less it will appear to move. Quasars tend to be ideal for this because they're all billions of light years away ... so far away that, for all practical purposes, you can pretend that they don't move... at all. Then you need to get an accurate position of your nearby the star (the one whose distance you want to know) and measure it's position relative to that really distant object as seen from your two different observing locations.



Parallax is just one of several ways to measure distances across space.

For *really* close objects, we actually can use radar. This is sometimes used for objects inside the solar system. There are other ways to work out location inside the solar system because we know how fast an object needs to move to maintain a given orbit around the Sun. In other words... get enough positional measurements over time and you can work out the orbit of an object and thus how far away it is located (as well as predict it's future positions).

Parallax measurements tends to be next on the list ... this can measure the distance to nearby stars. The closer the star, the more accurate the measurement. It used to be that Parallax could only provide a reasonable estimate for stars that are closer than 1000 light years... but with the ability to use satellites (no atmosphere to worry about) to take these measurements, the accuracy has improved over the years. I'm not quite sure what the current accuracy is.

"Standard Candles" is another method... this method says that if you *know* the true luminosity of an object, you can use that information to compare agains the perceived brightness (the light that ultimately does reach us here on Earth) to work out how far away that object is located. Cepheid variable stars are one type of standard-candle. Type 1A Supernovae are another standard-candle. (Cepheid variables were used to work out that the "Andromeda Nebula" was actually the "Andromeda Galaxy" ... prior to 1925, astronomers though the galaxy was just a gas nebula ... and hadn't realized it was actually a massive galaxy much much farther away. Edwin Hubble identified a handful of Cepheid variable stars in the "nebula" and determined (based on standard candles) that it *had* to be MUCH farther away than anyone had ever realized. This discovery blew open the doors on the size of the universe.
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