Saturday, August 13, 2011

The star Arcturus, or Hokule'a

In the previous post, we took a brief look at the amazing wayfinding prowess of the mariners who steer the double-hulled ocean-going canoe Hokule'a on voyages from the islands of Hawaii to destinations as far away as Tahiti, Aotearoa (New Zealand), Rarotonga, Nukuhiva, the West Coast of North America including Alaska, the coral atoll of Satawal in the Caroline Islands, and Japan, and which is scheduled to undertake a circumnavigation of the globe beginning in 2012.

Hokule'a is named for the star Arcturus, which is called Hokule'a ("Star of Gladness" or "Star of Joy") in Hawaii and Polynesia. Arcturus has added significance to Hawaii and celestial navigation such as that used by the ancient Polynesians because of its location in the celestial sphere at a declination of +19° 10' 56", which indicates the circle that it traces each day in relation to the celestial equator (positive declinations are towards the celestial north pole from the celestial equator, while negative declinations are towards the celestial south pole from the celestial equator).

This is significant to the Hawaiian Islands because they are situated on the globe at a latitude between just under19° north latitude and just over 22° north latitude. Because of this fact, Arcturus (or Hokule'a) is at zenith for observers at 19° north latitude, which includes observers in Hawaii. To understand why this is, think of the north star, which is at a declination of +90° (the celestial equator is designated as 0° declination, just as latitude on earth is measured from the 0° line of the terrestrial equator).

At the north pole on earth, which is 90° north latitude, the north star will be directly overhead. (actually, the earth's current north star is located at just over +89° declination, so an observer at the north pole would actually observe it making a tiny circle around the true celestial north pole). As an observer moves further south from there, the north star will begin to sink towards the horizon. After moving 10° south from the north pole, to 80° north latitude, the stars that will cross the zenith for an observer at that latitude will be those which are situated at a declination of +80°. Such as star will not stay directly overhead all the time (the way the north star would stay directly overhead for an observer at the terrestrial north pole), because it will make a circle in the sky as the earth turns. The circle made by a star at the same declination as the observer's terrestrial latitude will be a circle that goes through the zenith point of that particular location on earth.

For Hawaii, Arcturus moves along a circle which passes directly overhead, through the zenith, once every twenty-four hours (well, actually four minutes earlier each day, due to earth's progress around the sun, as explained in this previous post). For an outstanding discussion of the way that traditional wayfinders use the rising and setting of the stars to navigate across the open ocean without compasses or other modern equipment, see the series of web pages on the Polynesian Voyaging Society website (this page describes "Holding a Course" and contains diagrams showing the rising point of Hokule'a / Arcturus in the section entitled "Steering by the Stars" almost halfway down the long page; other web pages relating to Wayfinding can be located using the links in the menu along the right-border area near the top of the long web page).

If you are not familiar with the star Arcturus or Hokule'a, it is actually very easy to find throughout the year. Arcturus is the fourth-brightest fixed star in the sky (not counting the sun, which is not a "fixed star"), and the brightest in the ancient constellation Boötes, or "The Herdsman" (see illustration above). It is also known as Alpha Boötis.

To locate Arcturus, simply follow the "arc" of the handle of the Big Dipper (the final two stars in the Dipper's handle appear in the star map above, connected by a purple line; the rest of the Dipper is located in the direction of the words "Big Dipper" at the top of the diagram). Arcturus has an orange-red glow.

The rest of Boötes is easy to see, and it is a large and interesting constellation that will become increasingly familiar as you look for it in the sky each evening. This is a case in which the conceptual outlines created by the great H.A. Rey are particularly valuable: his outlines (added to the diagram above as red lines) make the constellation far easier to spot and far more memorable than do the flowery allegorical diagrams used in previous centuries (for an example of one of those for Boötes, see here) or than do the geometric and abstract outlines used by many modern star books and websites (for an example of the typical geometric outline of Boötes, see here).

In the outline created by H.A. Rey, the Herdsman is smoking a long-stemmed pipe which points towards the Big Dipper and actually comes very close to the Dipper's handle. You can see it on a dark night (the stars in the "pipe" are very faint).

Now you know more about Arcturus, or Hokule'a, the namesake of the famous vessel of the Polynesian Voyaging Society, and about the way its location in the sky relates to the location of the islands of Hawaii.

Below is a video of the great Israel Kamakawiwo'ole (1959 - 1997) singing "Hokule'a Star of Gladness."