Using the stars to make landfall

This month we focus on the third and final skill set essential for celestial navigation. After orienting the canoe and determining the canoe’s position at sea, the final challenge becomes making landfall.

Last month, we wrote about two ways of determining latitude, measuring the altitude of Hokupa‘a (North Star) and dead reckoning, an algebraic formula for estimating distance (speed x time = distance traveled). As you approach the equator you will lose sight of Hokupa‘a, as it will begin to get obscured by the clouds, and once you sail into the Southern Hemisphere, it will sink below the Northern Horizon and no longer be visible.

TWO GOOD stars to use to determine latitude in the Southern Hemisphere are found in the bucket of the Little Dipper: Hokumau (Pherkad) and Holopuni (Kochab). These stars have altitudes of 16 degrees and 18 degrees when they are in meridian (above the north celestial pole) at the equator. As you sail south in the Southern Hemisphere, the Northern Hemisphere stars get lower to the horizon and the Southern Hemisphere stars climb higher into the night sky. To determine southern latitude you measure the altitude of the 16-degree and 18-degree stars when they are in meridian, using the hand calibration technique described in last month’s column. If you measure the 16-degree star at 14-degree altitude, and the 18-degree star at 16-degree altitude then: 16 degrees – 14 degrees = 2 degrees latitude, and 18 degrees – 16 degrees = 2 degrees latitude.

You can continue this process of subtracting altitude as you sail in a southerly direction on your star compass until you get to a measurement equivalent to 14 degrees south, the limit to the range for using these two stars to measure latitude. At this point, these stars are so close to the northern horizon that they become difficult to see.

About the time you lose sight of Hokumau (16 degrees) and Holopuni (18 degrees) you should be crossing the northern boundary of the Tuamotu Archipelago, a band of 75 coral atolls spanning 900 miles. This is an exciting milestone for voyagers in the South Pacific. For several days prior to arriving at the edge of the northern boundary of the Tuamotu’s, you should have been glimpsing the Manu-O-Ku (white tern), a definite sign that you are approaching land. The sight of flotsam — natural terrestrial debris such as coconuts, branches, and leaves — and the appearance of the Noio (black noddy) bird, are other clues that landfall is only hours away.

LARGE anvil-shaped thunderclouds sometimes form over the interior lagoons of coral atolls. The bottoms of these clouds absorb the deep blue hue of the lagoons, and a trained eye can spot their characteristic blue tint. As you sail leeward of the atolls, before land appears, you should notice a quieting of the seas and a calming of the winds, along with a consistent large wave wetting the bow of the canoe. When the bow dries and that particular large wave diminishes, you are within 5 miles of sighting land.

Once the navigator finds the Tuamotu Archipelago, the navigation for that particular leg is over, and Tahiti is just 240 nautical miles away, or a short oceanic crossing of a day and a half, on a heading of Manu Kona, or southwest, on the star compass.

Special events in March

March 20 marks the vernal, or spring, equinox when the sun rises exactly in the east and sets exactly in the west. In the Northern Hemisphere this is celebrated as the first day of spring. Despite the name “equinox,” however, this day does not actually have equal parts of daytime and nighttime. In Honolulu, March 15 will actually be the day closest to having equal lengths of day and night, with the daytime lasting 12 hours and 1 minute.

March will have two full moons, the first one occurring on March 1 and the second on March 31. When this occurs the second full moon is often referred to as a “Blue Moon,” though the moon’s color does not change.

On the evening of March 15, Mercury will be at a unique position in its orbit known as Greatest Eastern Elongation. From Earth’s perspective, this is Mercury’s farthest position from the Sun and the best night to view the planet. Note, however, that Mercury will still be setting early in the evening and will disappear from our sky by 7:45 p.m.

March night sky

Rising out of Manu Ko‘olau, the northeastern horizon, look for the recognizable shape of Nahiku, famously known as the Big Dipper.

This constellation of seven stars is a familiar sight across the Northern Hemisphere. The two bright stars in the front scoop of the dipper point directly to Hokupa‘a (North Star).

As a fun challenge, look at the second star from the end of the handle of the dipper. If you have good vision you may be able to pick out two, or even three stars nearby. The brightest of these is Mizar, next to it will be the fainter star Alcor, and between Mizar and Alcor, those with truly excellent eyesight can make out the faint Ludwig’s Star. In earlier times these stars were commonly used as a vision test.

As we enter the spring months, the familiar shape of Kaheiheionakeiki (Orion) will be high in our western sky during the early evening. This shape is quite famous around the world as it is composed of bright stars and lies on the celestial equator, meaning that no matter where you are on Earth, it will be visible in the sky.

To the north and west of Kaheiheionakeiki will be the distinctive star cluster of Makali‘i, also well known as the Pleiades, or “Seven Sisters.” This is one of the closest star clusters to our solar system at only 444 light years away.

Throughout March, sunrise occurs around 6:30 a.m., giving early risers a beautiful view of the sky. During these early morning hours, the planets of Jupiter, Mars, and Saturn will be rising in the southeast, lined up nicely in front of the central bulge of the Milky Way.

Chad Kalepa Baybayan is navigator-in-residence and Emily Peavy is planetarium technician support facilitator at the ‘Imiloa Astronomy Center of Hawai‘i , a center for informal science education on the UH-Hilo campus, showcasing astronomy and Hawaiian culture as parallel journeys of human exploration.