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As spring turns to summer, the months of May and June have provided Polynesian mariners the best season for making a Tahiti-to-Hawaii Pacific passage. The Southern Hemisphere is entering its winter season and cooling ocean temperatures. The Northern Hemisphere is about to enter its summer season when ocean temperatures rise, providing the energy and conditions for hurricanes to form off the western coast of Mexico. It is wise that mariners conclude oceanic passages between Northern and Southern hemispheres by July.
The steady tradewinds, oceanic swells and star field all move toward the canoe from the eastern horizon as you sail north and away from Tahiti. Keep the rising star field on the right and the setting star field on the left. After 10 days of sailing on a northward tack you should be able to make out Hokupaa (Polaris), the North Star, just above the northern horizon confirming that you are now in Northern Hemisphere waters. As you sail between Southern and Northern hemisphere oceans, you will encounter an area of convergence.
Doldrums
Known as the Inter Tropical Convergence Zone (ITCZ), and more commonly by its popular name, the doldrums, this is a band of ocean where the east-southeast, Southern Hemisphere winds and east-northeast, Northern Hemisphere winds converge.
The ITCZ can span a distance of 300 nautical miles and is typically marked with a high ceiling of cloud cover and rainy weather. The trade winds diminish in the doldrums and are replaced with light and variable winds. The oceanic current flow in the ITCZ runs opposite to the westerly flow of the northern and southern waters between Hawaii and Tahiti. In the ITCZ there is an oceanic counter-current that pulls the canoe to windward; it’s a kind of gift from the sea gods that keeps the canoe on a windward approach to the Hawaiian Islands.
Hokule‘a employs the “Upwind” system of sailing, that is, we sail to the latitude of the target and keep the vessel upwind of the island. We can determine latitude accurately by measuring key stars as they pass through the meridian, an imaginary line that runs between the north and south celestial poles. The meridian would mark the highest point on a stars trajectory as it moves between the eastern and western horizons. Once the navigator is confident that he or she is at the correct latitude and upwind of the target they will initiate a downwind search for land.
Southern Cross
In May and June, Hanaiakamalama, the Southern Cross, is in optimum position for determining accurately the latitude for the Hawaiian Islands. By July, Hanaiakamalama is in the direction of the setting sun and by nightfall it is beyond the meridian and too late to measure. The distance between Kaulia, the top star of the Southern Cross, and Kamolehonua, the bottom star of the Southern Cross, is 6 degrees when it crosses the meridian or when one star is above the other. When they are equidistant to the horizon, that is, when the top star and the bottom star of Hanaiakamalama is equal to the distance of the bottom star to the horizon at meridian, you are at center latitude for the Hawaiian Islands. With my palm in hand position, 6 degrees is about 2-1/2 fingers between the bottom star of Hanaiakamalama and the horizon at meridian.
If the navigator is confident, he or she will initiate a downwind search for land. On our star compass, we would keep the bow of the canoe in the downwind position between the star houses Aina Kona and Aina Hoolua. The center of the Hawaiian Islands should be a line that runs east to west a few miles south of La Perouse Bay on the island of Maui.
June’s night sky
During the evening of June 27 the beautiful planet Saturn will be rising alongside the full moon. On this evening Saturn will be at opposition. Opposition refers to when an outer planet (Mars, Jupiter, Saturn, Uranus or Neptune) is on the opposite side of the Earth from the sun.
When a planet is at opposition it is at its closest physical position to Earth, it will look somewhat brighter in the sky, and will rise at the same time as the sun thus remaining in the sky the whole night. Saturn’s opposition this year happens to be on the same night as the full moon thus it will accompany the moon in the sky the whole night.
June 21 marks the summer solstice, the day when the Northern Hemisphere is at its furthest tilt toward the sun. On this day the Northern Hemisphere experiences its longest period of daylight; the day length for Honolulu on the solstice will be 13 hours and 25 minutes. As we are closer to the equator the day length changes between the summer and winter solstices is less dramatic. For example, Portland, Ore., will experience a 15-hour-and-41-minute day on the solstice.
Evening
Through the month of June the bright planets, Venus and Jupiter, will clearly stand out in the sky. Venus will be near the Manu hoolua (northwest) horizon. While Jupiter will be high in the hema (southern) sky. Venus and Jupiter are some of the brightest objects that we ever get to see in the sky. Observers using a telescope to look at the planets will be able to see Venus pass through phases (similar to the phases of the moon) as well as some of the moons around Jupiter.
On a dark, clear night observers with a careful eye will be able to see a faint blurry ball of light to the north and east of Hanaiakamalama (Southern Cross). This ball of light is Omega Centauri, a globular cluster; globular clusters are unique in and of themselves as they move in large orbits around the center of the galaxy in the halo region (as opposed to orbiting within the spiral arms).
Omega Centauri is the largest globular cluster orbiting the Milky Way as it contains approximately 10 million stars and is 150 light years in diameter. Some globular clusters, including Omega Centauri, are thought to be the core of a dwarf galaxy which came too close to the Milky Way; the Milky Way then absorbed most of the mass of the dwarf galaxy but left the core orbiting in the halo of the galaxy. Omega Centauri is also believed to have an intermediate mass black hole lurking in the center of this unique object.
Early mornings
Early risers looking at the sky before 5 a.m. will see a different view of the sky. The rusty red planet Mars will be high in the southwest chasing the still visible Saturn into Manu Kona (the southwest horizon). High overhead the three bright stars of the Navigator’s Triangle, also known as the Summer Triangle, and rising in the northeast will be the “W” shape of Iwakelii (Cassiopea).
Highlights include Hanaiakamalama (the Southern Cross). In the evening May sky, Hanaiakamalama is in optimum position for determining accurately the latitude for the Hawaiian Islands. When the distance from the bottom star to the horizon matches the distance from the top star to the bottom star, you are at center latitude for the Hawaiian Islands. However, this can only be measured when the cross is standing straight up, as it will around 8 p.m. in May.
June 2018 Sky Chart by Honolulu Star-Advertiser on Scribd
Chad Kalepa Baybayan is navigator-in-residence and Emily Peavy is planetarium technician support facilitator at ‘Imiloa Astronomy Center of Hawai‘i at the University of Hawaii-Hilo.
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