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Thursday, 17 March 2011

Planetary Movements, Spring Tides and Equinox

Joseph Proskauer gives us an equinoctial update on the movements and visibility of the planets and the effects of full Moon and spring tides.

March brings Mercury's best evening apparition of the year (worthwhile, as few of us have knowingly seen Mercury):

Mercury also happens to spend the week through Friday March 18th within 5° of Jupiter. (That's half the width of your fist at arm's length.)
All you need is a clear evening and a viewing site with an good view down to the west horizon. Note the spot where the Sun sets, and then start scanning above (and slightly to the left) of there for Jupiter. Jupiter may be visible immediately if the air is very clear, but it will be more obvious 15 or 30 minutes later when the sky is darker (though Jupiter will also be lower).

Once you've found Jupiter, look for Mercury near it. These are by far the brightest objects in that part of the sky, so there's no chance of mistake. (The only possible confusion would be from airplanes; but airplanes will shift within a minute or two, while Jupiter and Mercury stay put except for their gradual descent toward the horizon.)

In a remarkable coincidence, both planets pass through perihelion this week (closest to the Sun in their orbits). Mercury has a perihelion every 88 days, but Jupiter has one only every 12 years.

Mercury appears higher each evening in March until the 22nd, while Jupiter appears ever lower. So by the end of that period, Mercury may actually be the more obvious of the pair, despite the fact that it's slowly fading. Starting around March 25th, Mercury plunges back toward the Sun, fades rapidly, and soon becomes hard to locate with the unaided eye.

Equinox -- March 20th:

One of the two days each year when the sun rises truly in the east, and sets truly west.

What shape does the tip of a shadow trace on the Equinox? Try marking the tip of any shadow (post, roof, tree, or standing stone) every hour or two during the day.

Equinoctial events at Cairn Holy:

Full-moon rise: Around sunset Friday March 18 (before 5:30 PM at Cairn Holy).

Equinoctial sunrise -- the heart of the year at Cairn Holy:
Closest (most perfect) sunrise is Sunday March 20th (shortly after 6:30 AM at Cairn Holy -- allowing for eastern ridge). But all sunrises that week (e.g. Friday, Saturday, Monday, Tuesday) will show the general phenomenon. There are further interesting events which follow throughout the morning.

Midday: Cairn Holy's midday alignment is visible any sunny day; local noon is currently 12:28 -- progressing to 12:26 for the equinox weekend.

Equinoctial sunset: before 5:30 PM (at Cairn Holy).

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