# Size comparison Sun/Mars- Sirius A/B

Hi i was listening to you speak and you mentioned mars is 205 times bigger than the Sun just like Sirius A/B ? where are you getting data from because when I did the math Mars is 205.1 times bigger than sun
indeed but Sirius B is 206.12 so its not really identical did you just avg the two to 205 ? or where did you get the numbers for A and B perhaps i got my numbers wrong for them ? can you please post the numbers you used to get 205 as well?

Hello Gnostic Troll,

Firstly, please know that there isn’t really a firm consensus about the exact diameters of Sirius A and Sirius B (nor even about the exact diameters of the Sun and Mars). You may find slight discrepancies depending on which source you happen to find - and even modern academic papers are constantly updating their data. So yes, you could say that I have ‘averaged’ these slightly conflicting estimates (taken from various sources) for my own calculations.

As you can read in Chapter 6 of my book:

Diameter of SIRIUS A: 2 390 000 km / Diameter of SIRIUS B: 11684.4 km

Ergo: Sirius B’s diameter is 0.4888 % that of SIRIUS A

Diameter of our Sun: 1 392 000 km / Diameter of Mars: 6792.4 km

Ergo: Mars’s diameter is 0.488 % that of the SUN

That’s right — 0.4888% versus 0.488% … a proportional difference of barely 0.0008%!

We may also put it the following way (which may be easier to memorize):

Sirius A is about 205 times larger than Sirius B - and our Sun is about 205 times larger than Mars.

Note that 100 / 0.4888 = 204.58 and that 100 / 0.488 = 204.91

So my approximate “205 X larger” estimate (which I usually state in audio podcasts) is only a way to simplify the concept for the listener. To nitpick on this issue isn’t really going to make much difference, is it?

Then could you please reference where did you get the data for Sirius then,… The : 2 390 000 km / Diameter of SIRIUS B: 11684.4 km

sorry the reason i ask is because with these things being accurate is important as you cant just pick one source for the diameter and ignore the other sources but then pick the source that fits the 205.

as i mentioned using a different source can give you the difference of 203 vs 206
i know its small but i think its important to then mention in the book that you are averaging out and its not 10000% identical numbers in both… going from 203 to 206 for example is not ‘’ a proportional difference of barely 0.0008%!‘’’ as you stated.

Dear Gnostic Troll,

Please know that, in astronomy, there is something called the "uncertainty range’ (or ‘error range’). This is expressed (in numerical notation) with a ± symbol. For instance, Sirius B’s diameter is reckoned to be anything between 0.00815 and 0.00865 times that of our Sun. Either of these two lower & upper bounds could be the ACTUAL / CORRECT size of Sirius B. No one really knows for sure :

“Sirius B Radius has been calculated as being 0.008 times [the size of] the Sun. The Sun’s radius is 695,800km, therefore the star’s radius is an estimated 5,844.72.km. If you need the diameter of the star, you just need to multiple the radius by 2. The error range for the radius is between 0.00815 and 0.00865.” Source. Sirius B (Alpha Canis Majoris B) Star Facts (Distance, Age, Mass, Colour, Radius & more) - Universe Guide

So, for the purpose of this discourse, let’s say that the ACTUAL / exact diameter of Sirius B is 0.00835 X that of the Sun. Note that this (slightly ‘arbitrary’) “0.00835” value is a quite reasonable choice - as it is well within (and almost in the middle) of the 0.00815 > 0.00865 uncertainty range.

0.00835 X 1 392 000km = 11623.2 km

Next, let’s use the estimated figure of 2 380 700 km for the diameter of Sirius A - as stated in this source: “Diameter of Sirius A : 2 380 700km”

We see that: 2 380 700km / 11623.2km = 204.823 (i.e, Sirius A is about 204.8 X larger than Sirius B)

Let’s now compare the diameters of the Sun (1 391 400 km) and Mars (6792 km) as published in these JPL/ NASA tables. Source: https://www.jpl.nasa.gov/edu/pdfs/scaless_reference.pdf

1 391 400 km / 6792 km = 204.85 (i.e. the Sun is about 204.8 X larger than Mars).

We may therefore justly say that the relative diameters of the Sun & Mars and Sirius A & B are proportionally (near-)identical - roughly at a 205:1 ratio. 1 Like