“Guam’s Tropical Skies” with UOG’s Pam Eastlick


Guam – A look at “Guam’s Tropical Skies” is brought to you by University of Guam Planetarium Director Pam Eastlick

For all the latest on Guam’s Tropical Skies go to the Planetarium’s website at: www.guam.net/planet

INFORMATION FOR THE WEEK OF :  June 18 – 24, 2012

I told you last week to go out and have a look at the early morning sky for a conjunction of Jupiter, Venus and the thin crescent Moon. Well, if the Moon was a thin crescent in the dawn sky last week, that means that new Moon is this week (it’s on Wednesday) and that thin crescent will soon appear in the evening sky.

If you watch one of our gorgeous sunsets and the clouds cooperate, on Friday night you’ll be able to see that sliver-thin crescent Moon keeping close company with another of the planets. Find the Moon on Friday night and that bright star two fist-widths to the right and one fist-width down isn’t a star; it’s Mercury. At one hour after sunset, Mercury will be a little less than two fist-widths above the western horizon.

So . . . just when is one hour after sunset? Well, that changes of course, depending on the season and your latitude. We all know that the days are the same length at the equinoxes in March and September. The shortest day is the first day of winter and the longest day is the first day of summer. Right? Well . . . maybe. You’ve probably noticed if you’ve ever spent time in temperate latitudes like the mainland US, that the days are very short in winter and very long in summer. The effect is more pronounced the farther north you go. In most of northern Alaska, the Sun doesn’t set in the summer and it’s daylight all the time. In the winter, the Sun never rises and it’s always dark. This difference in daylight length is caused by the fact that the Earth’s rotation axis is tilted in relation to the Sun. In northern hemisphere summer, the north pole of Earth is tilted toward the Sun. More light falls on the northern hemisphere and heats it up. This tilt causes the seasons. Here on Guam, we live near the middle and our part never tilts toward or away from the Sun very much. Because of this, our day length doesn’t change much either. We get about 11 hours of daylight in the winter and 13 hours of daylight in the summer.

Why am I talking about this? Well, Thursday, June 21st is the first day of summer in the northern hemisphere and it should be the year’s longest day, right? Well, not exactly. Summer solstice does occur on the 21st for us this year (unlike last year when it happened on the 22nd) but here on Guam the days don’t get longer and longer until 21 June and then start getting shorter again. We have more of a slide, when the Sun starts rising later, but also continues to set later for about a month. Sunrise was ‘stuck’ at 5:53 a.m. the earliest rise time of the year, for most of the first of June. On 11 June, the Sun rose at 5:54 a.m. and by the end of the month will rise at 5:57 a.m.

We started the month with sunset at 6:45 p.m. The Sun will continue to set later all month and it will set at 6:52 p.m. on the 30th. So exactly when is the longest day of the year? You’ve been enjoying it all month. The longest day started on the 7th because all the rest of the days in June are either 12 hours and 54 minutes long or 12 hours and 55 minutes long.

We may not have midnight Sun or even 14 or 15 hours of daylight here this week, but we do have a “longest day” that lasts for most of the month of June! Enjoy the extended solstice and the summer!