Astro-navigation cannot be regarded at present as one of the principal aids to air navigation. It suffers, by comparison with radio aids, in that it is slow in use and much more subject to interruption by bad weather. The average navigator needs at least 15 minutes to take and plot a two-star fix with present methods. This is quite a long time in the air, though not prohibitive with present-day aircraft, and in consequence astro-navigation is now used extensively only on long flights over sea or desert where radio aids are lacking.