How Do We Measure the Distance to a Star? | Hacker News Reader

How Do We Measure the Distance to a Star?

scientificamerican.com

74 pointsyaseen-rob27 comments

Parallax is barely the first rung of the cosmic distance ladder:

https://en.wikipedia.org/wiki/Cosmic_distance_ladder

It's a huge pet peeve of mine when the science part of this kind of edutainment media is either wrong, or (as in this case) dumbed down to the point of uselessness. The triangle diagram here is completely wrong, and yeah, it uses high-school trig, but c'mon--if it's something we all supposedly learned in high-school and your audience is high-school educated... Show the actual diagram and the actual math!

Scientific American isn’t aimed at professional scientists or STEM majors. Think bright middle class high school kids.

Back in the 1960's my grandfather, who didn't have a high school education (farm raised, 1903-1994) had a subscription to Scientific American and would read them cover to cover.

Starting when I was 7 or 8 he would talk about the articles with me while we were working on things, either in his shop or cutting timber or repairing things around the farm. He would ask probing questions about the subjects, look for implications, make connections to other things he knew, all the kinds of things you want a scientific mind to do. Despite a lack of formal education and wearing a pair of blue jean overalls and muddy wellington boots, with tools in his hands, he was a smart and erudite for his circumstances.

He gave me his entire library when I was in my early teens. The first program I ever wrote, Conway's Life, was taken after a Martin Gardner article I read there. I credit those talks with my grandfather, and the articles I read, with a large portion of my being drawn to STEM and seeing it as "something for me".

SciAm was once a great institution trying to spread knowledge of the explosion of scientific understanding that was happening at the time. I miss those days.

I used to read the Connections column by James Burke regularly but he quit writing it shortly after my family actually sprung for a subscription :(

Idk, there is some value in making the concept as accessible as possible. It makes the concept more believable to even the lowest common denominator. The more cognitive energy you have to spend on something, the harder it is to consume and believe.

With an attitude like that, I don’t know why any kid would be inspired to learn math and science.

Gell-Mann Amnesia in full force. [0] But, consider that there's an animation budget at work here.

[0] https://www.goodreads.com/quotes/65213-briefly-stated-the-ge...

Surprised it wasn't mentioned in the video, but this is where the term 'parsec' come from. A parallax angle of 1 arc second means the object is 1 parsec away (3.26 light years).

They have some beautiful animations, but can any astronomers confirm if people actually do it this way?

From class I remember that you can also determine the distance by measuring the coherence of the light coming from the star at two distant points on the order of 100m. In theory this would require more equipment, but has the obvious advantage of taking less than 6 months to get the results.

https://en.wikipedia.org/wiki/Cosmic_distance_ladder - "At the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. ... The most important fundamental distance measurements come from trigonometric parallax."

Also highly recommend Terry Tao's talk on the cosmic distance ladder:

https://terrytao.wordpress.com/2010/10/10/the-cosmic-distanc...

In addition to the annual parallax of the star being measured, you also need the diameter of the Earth's orbit, which was also first measured by parallax (together with Kepler's laws, giving the ratio of planets' orbital sizes from their periods): When Mars came close to Earth in 1672, simultaneous observations by Giovanni Cassini (in Paris) and Jean Richer (in French Guiana), comparing where Mars appeared relative to background stars, yielded a value of an astronomical unit that was about 7% higher than the modern value. [1]

Subsequent measurements of the dual transits of Venus in 1761 and 1769 refined the figure (Lalande, 1771), again using parallax, in a method proposed by Halley. Current measurements are based on radar.

[1] http://curious.astro.cornell.edu/physics/62-our-solar-system...

Former astronomer here. Yes, parallax measurements are absolutely the foundation of almost all distance measurements in astronomy.

The obvious advantage of using parallax is that you effectively use for the measurements an "instrument" of the size of 300 million kilometers (190 million miles). Which is really needed when the nearest star is 41 trillion(!) km away.

Parallax is not the only way. Cepheid variables are also used, I think more so than parallax, since parallax is only accurate for somewhat nearby stars.

https://en.m.wikipedia.org/wiki/Cepheid_variable

Yes, but Cepheids are calibrated, fundamentally, by parallax. (I believe that no Cepheids were actually close enough to be calibrated directly by parallax, at least until Gaia, so they had to be calibrated by other techniques which were, in turn, calibrated by parallax.)

Here is a 9 videos playlist (that I find helpful) about the methods astronomers use to measure the cosmic distances: https://www.youtube.com/playlist?list=PLEE8A6CB118FADED4

The author was my undergrad classmate. She did biology or biological engineering IIRC, glad to see she’s hitting the big time.

https://en.wikipedia.org/wiki/Stellar_parallax

How accurate is parallax, given that stars bend light?

It's accurate to within 100 light years. See https://starchild.gsfc.nasa.gov/docs/StarChild/questions/par....

"Accurate to within 100 light years" isn't really a meaningful statement. The page you linked to is a simplified explanation that says the method is only accurate enough to be used for stars within 100 light-years of Earth, which is approximately true. We have measurements of more distant stars, but they become increasingly inaccurate with distance.

To be more precise, the relative error of a parallax distance measurement is proportional to both the measurement error and the distance. For example, the Hipparcos mission measured parallax with an accuract of roughly 0.001 arc-seconds. That corresponds to a 0.1% error at a distance of 1pc (~3 ly), or 1% error at a distance of 10pc, or 10% error at a distance of 100pc, and so on.

Note also that the page you linked was written sometime around 2002; since then, more sensitive measurements have been done, e.g. https://arxiv.org/abs/1401.0484v1

The only star that matters in terms of bending light affecting the measurement of parallax is our own Sun, and that's taken into account with modern survey missions (such as Hipparcos and GAIA).

Understanding this is not a high priority for me. Just sayin'. :)

Glad you could still find the time to tell us.

The simplest method would be to send a spaceship there and back and calculate distance based on fuel usage.

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