A Star to Steer By
When we think of celestial navigation, we imagine the master of an old sailing ship taking sights of the stars with a sextant, chronometer, wearing a pointy hat. To us it seems an arcane art of a bygone era, best not spoken of in polite company.
In this installment of “Bye, Bye Birdie”, we’ll be talking about the much older art of literally steering by the stars, both as a means of checking our compasses and as simple aids to navigation, like lighthouse beacons burning steadfastly above us, whether in open ocean or urban harbors.
We don’t need to be veteran stargazers to utilize the sky in this way. From just three easily identifiable constellations, we can find our way anywhere in this world. I’ll present three ways of using the stars for guiding our vessels, requiring no special equipment or degrees in astronomy.
One of the most important uses of
celestial bodies in modern navigation is the ability to externally verify our compasses (magnetic and electronic) when land is out of sight. With the aforementioned ephemerides (a table giving the coordinates of celestial bodies) and some math, we can use any celestial object for navigation at any time. However, there are a few cases that require no tools, tables, or math. These instances fortuitously rely on two of the best known and easiest to find constellations in the sky—The Big Dipper and Orion.
Since the stars are not always visible, the first step, when we can, is to determine our compass error. Once we’ve established that our compass card is in order, we’ll be delighted to then ignore our compass altogether and simply steer our vessel by the night sky.
Our north star is situated almost (but not quite) directly over the north pole. At the latitudes we are likely to be cruising, it is rarely more than 1.5° azimuth from true north, and below 40° north latitude, never more than 1°. This makes Polaris an excellent and simple object with which to check our steering compass and verify (or create) our magnetic deviation table. In an emergency, we may utilize Polaris in this way with no mathematical calculation, other than the simple arithmetic needed for any compass check.
To find Polaris, first find the Big Dipper. The Big Dipper is also referred to on star maps as Ursa Major, the Big Bear. All of the stars in the Big Dipper are roughly the same brightness. Following the two stars on the right side of the constellation’s famous dipping cup as pointers, Polaris is the next star along that line that we find that carries roughly the same brightness.
From any place more than a few degrees north of the equator, Polaris is visible in the night sky, essentially due north, all night long, on any clear night at any time of year. It is hard to imagine a simpler and more useful tool in our navigation toolbox.
The constellation depicting the hunter, Orion, straddles the equator, so at least part of it is visible from every single location on earth from November through February. At other times it is partly or fully obscured by the sun, but for four months or so out of every year Orion is one of our most prominent and recognizable celestial companions.
Probably the most recognizable feature of Orion is his belt of three stars in a line, and these also happen to be our most useful tool for celestial navigation. Orion’s belt, and particularly the westernmost of the three stars, Mintaka, falls almost exactly on our equator. Because this is true everywhere in the world every day whether we can see it or not, Mintaka rises due east and sets due west. Like Polaris, this is precise enough for us to use for compass checks without any additional math.
In the tropics, Mintaka will be nearly due east or due west (unless directly overhead) all night long, but the farther we are from the equator, the less time we have to observe Mintaka at that bearing. Nonetheless, it is also a useful tool for us.
Many star maps do not identify Mintaka as such, or at all. It is also called Delta Orionis, and on star maps may simply be identified as a Greek letter “d” in that constellation. It doesn’t matter much. If you are in the northern hemisphere, Mintaka is the star on the far right of Orion’s belt; if you are in the southern hemisphere, it is on the far left.
THE SOUTHERN CROSS
Whereas navigators in the northern hemisphere are blessed by the ever-present Polaris, the sky above our south pole is practically devoid of any bright stars.
We do, however, have the beautiful Southern Cross.
The Southern Cross (also called Crux) always points to the south pole, but in order for that to help us in this context, it needs to be more or less vertically oriented. Otherwise, it’s a bit like if the Big Dipper pointer stars pointed at nothing; you only know that south is that general region of the sky. But it is still better than nothing.
Be careful to not confuse the False Cross (an asterism formed by stars from two constellations) with the real one. Crux is easily identified by its own bright pointer stars—Hadar and Rigel Kentaurus—better known as Alpha Centauri, our nearest stellar neighbor.