SEAMANSHIP

A Star to Steer By

Passage Maker - - Contents - Robert Reeder

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.

PO­LARIS

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.

ORION’S BELT

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.

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