How many hours of dark do we have each night? And how many hours of “sunlight” at night? (The Moon shines by reflected sunlight.) Each year the Sun swings north and south along the ecliptic, crossing the celestial equator at the equinoxes, from its northernmost point (+23.5º declination) at the June solstice and to its southern limit (-23.5º) on the December solstice. This variation determines the hours of sunlight each day according to the equation cos(H) = tan(declination) * tan(latitude), where twice the quantity H equals the hours above the horizon. On Delmarva, the seasonal sunlight ranges from 14.8 hours in summer to 9.2 in winter. The Moon too rides near the ecliptic, but rather than taking a full year, it slides completely around the sky every 29.5 days. Thus moonlight follows nearly the same pattern as sunlight but cycles over 12 times as often. The resulting pattern of seasonal sunlight and monthly moonlight give rise to windows of deep night familiar to deep-sky observers.

Mathematically, the Moon’s motion is a complicated affair. The orbit is elliptical and perturbed by the Sun’s gravity. More significant for its rising and setting, the orbit is inclined to the ecliptic by more or less 5º. Over a 19-year cycle, the Moon’s extreme declination varies from about +28.7º to -28.7º (that is, Earth's obliquity plus the moon's inclination).These extremes will next be reached in 2006 and will add nearly an hour of moonlight on certain nights at our latitude. In 2004, moonlight ranges from 9 to almost 16 hours when parallax and refraction are taken into account.

This week, the Moon joins a truly fantastic sight in the evening sky. All five bright planets will be visible: from Mercury low in the west, to brilliant Venus, then Mars (in Taurus), Saturn (Gemini) and finally Jupiter (Leo), and these planets will be joined by the cresent through gibbous Moons through early April. Because of its inclined orbit, the Moon will slide south of Mercury and Venus, in front of Mars (an occultation for Alaska on the 25th) and north of Saturn and Jupiter. In a repeat performance beginning in December, these five planets will be arrayed in the morning sky—this time in the same order (from low in the east to near the meridian) as their distance from the Sun. The Moon will join them starting on 28 December. To record this event, you’ll need a wide angle lens or to create a photo-mosaic of separate images: these planets will span over 130º of sky. In the interim, Venus passes between Earth and the Sun in inferior conjunction. On June 8th, the Sun and Venus align precisely enough for the first transit of Venus since 1882. Mark this date on your calendar.

These are all great planetary photo-ops. What is prettier than a crescent hung low in the twilit sky alongside a brilliant evening star? Or a full moon rising from within the pink of the Earth’s shadow? The Moon and moonlight were made for the the aspiring astrophotographer. Keep shooting!

This topic was prompted by a discussion on the Delmarva Star Gazers Yahoo! Group. For more information on planetary groupings, see Jean Meeus’ More Mathematical Astronomy Morsels, p. 347 (2002, Willmann-Bell).  Moondark is written by Doug Miller, published at  the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 21 March 2004. Text and images copyright © 2004 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.