Miaplacidus and Latitude Estimations

By sailing south from Hawai’i to 17 degrees N latitude, then heading west, the navigators plan to sight Johnston Atoll. How will they know when they are at 17 degrees north latitude?

To estimate their latitude, the navigators observe the altitude of stars above the horizon as the stars journey from east to west across the night sky. Midway on their journey, the stars cross the meridian, an imaginary line that arches overhead on the celestial dome from due north to due south.

At a given latitude, stars transit the meridian at given altitudes.

At 17 degrees N, for example, the latitude of Johnston Atoll, Miaplacidus transits the meridian at 3 degrees above due south, aligning perpendicularly to the horizon with the stars Turais and Al Suhail directly above it. At this time of the year (end of January) this transit takes place after midnight (and at the same location, a little earlier each night).

Miaplacidus, in the constellation Carina, the Keel, is generally not visible in Hawai’i, which is located between 19-22 degrees N latitude. For each degree north we travel, a star transits the meridian above the southern horizon one degree lower; for each degree south we travel, the star transits the meridian one degree higher. So at 20 degrees N in Hawai’i, Miaplacidus transits at 0 degrees, or at the horizon, and at latitudes above 20 degrees N, it is below the horizon and cannot be seen.)

As the canoes head south toward Majuro, at 7 degrees N, Miaplacidus would transit the meridian one degree higher for each degree of latitude they travel south. Thus at the latitude of Majuro, Miaplacidus would transit at 13 degrees above due south. Observing this change in altitude each night (if the sky is clear enough to allow the navigators to see the stars), the navigators estimate their southerly progress and know when they are at the approximate latitude of Majuro. Sailing west (assuming they haven’t sailed east of Majuro), their course should lead them to Majuro.

Generally, the closer to the horizon the transit star is, the better the estimate of latitude. The navigator measures the altitude of celestial bodies with his outstretched hand, different parts of his hand calibrated to different altitudes.

Viewing Miaplacidus in an Online Planetarium

To see Miaplacidus transiting the meridian in an online planetarium, log onto Astroviewer at http://www.astroviewer.com/interactive-night-sky-map.html.

Click on “Start Astroviewer.”

Click on the “Location / City” button. Move the crossing lines into the Pacific Ocean just south of Hawai’i by clicking on that area, then use the arrow buttons to change the latitude and longitude to the approximate location of the canoes (currently around 17 degrees north and 164 W).

Using the arrow buttons in the Time box, set the date to Jan. 31 2007, and the local time to 28 minutes past midnight (00:28).

Use the slide on the left side of the viewing box to increase the magnification of the viewing box / star chart. Use the slide on the button to scroll along the horizon to due south (S).

The constellation known as The Keel (Carina) should appear above S.

The red perpendicular line in the middle of the box is the meridian. The star just above the curved horizon line, on the meridian, is Miaplacidus. Click on the star for more information about it. Above Miaplacidus, near the meridian, is Turais, and above Turais is Al Suhail.

Changing the time (clicking on the time arrows for minutes and hours) will set the sky in motion and you can see Miaplacidus moving west and curving down toward the horizon, where it will set at about 2:45 a.m.

January 31, 2007 | PVS