Aristarchus's inequality

Aristarchus's inequality (after the Greek astronomer and mathematician Aristarchus of Samos; c. 310 – c. 230 BCE) is a law of trigonometry which states that if α and β are acute angles (i.e. between 0 and a right angle) and β < α then

${\displaystyle {\frac {\sin \alpha }{\sin \beta }}<{\frac {\alpha }{\beta }}<{\frac {\tan \alpha }{\tan \beta }}.}$

Ptolemy used the first of these inequalities while constructing his table of chords.^[1]

Proof

The proof is a consequence of the more widely known inequalities

${\displaystyle 0<\sin(\alpha )<\alpha <\tan(\alpha )}$ ,

${\displaystyle 0<\sin(\beta )<\sin(\alpha )<1}$  and

${\displaystyle 1>\cos(\beta )>\cos(\alpha )>0}$ .

Proof of the first inequality

Using these inequalities we can first prove that

${\displaystyle {\frac {\sin(\alpha )}{\sin(\beta )}}<{\frac {\alpha }{\beta }}.}$ 

We first note that the inequality is equivalent to

${\displaystyle {\frac {\sin(\alpha )}{\alpha }}<{\frac {\sin(\beta )}{\beta }}}$ 

which itself can be rewritten as

${\displaystyle {\frac {\sin(\alpha )-\sin(\beta )}{\alpha -\beta }}<{\frac {\sin(\beta )}{\beta }}.}$ 

We now want show that

${\displaystyle {\frac {\sin(\alpha )-\sin(\beta )}{\alpha -\beta }}<\cos(\beta )<{\frac {\sin(\beta )}{\beta }}.}$ 

The second inequality is simply ${\displaystyle \beta <\tan \beta }$ . The first one is true because

${\displaystyle {\frac {\sin(\alpha )-\sin(\beta )}{\alpha -\beta }}={\frac {2\cdot \sin \left({\frac {\alpha -\beta }{2}}\right)\cos \left({\frac {\alpha +\beta }{2}}\right)}{\alpha -\beta }}<{\frac {2\cdot \left({\frac {\alpha -\beta }{2}}\right)\cdot \cos(\beta )}{\alpha -\beta }}=\cos(\beta ).}$ 

Proof of the second inequality

Now we want to show the second inequality, i.e. that:

${\displaystyle {\frac {\alpha }{\beta }}<{\frac {\tan(\alpha )}{\tan(\beta )}}.}$ 

We first note that due to the initial inequalities we have that:

${\displaystyle \beta <\tan(\beta )={\frac {\sin(\beta )}{\cos(\beta )}}<{\frac {\sin(\beta )}{\cos(\alpha )}}}$ 

Consequently, using that ${\displaystyle 0<\alpha -\beta <\alpha }$  in the previous equation (replacing ${\displaystyle \beta }$  by ${\displaystyle \alpha -\beta <\alpha }$ ) we obtain:

${\displaystyle {\alpha -\beta }<{\frac {\sin(\alpha -\beta )}{\cos(\alpha )}}=\tan(\alpha )\cos(\beta )-\sin(\beta ).}$ 

We conclude that

${\displaystyle {\frac {\alpha }{\beta }}={\frac {\alpha -\beta }{\beta }}+1<{\frac {\tan(\alpha )\cos(\beta )-\sin(\beta )}{\sin(\beta )}}+1={\frac {\tan(\alpha )}{\tan(\beta )}}.}$ 

See also

• Aristarchus of Samos
• Eratosthenes
• Posidonius

Notes and references

1. Toomer, G. J. (1998), Ptolemy's Almagest, Princeton University Press, p. 54, ISBN 0-691-00260-6

External links

• Leibowitz, Gerald M. (PDF). Archived from the original (PDF) on 2011-09-27. Retrieved 2019-06-24.
• Proof of the First Inequality
• Proof of the Second Inequality