May 31, 2020

AstroEvent: A Pacific-Spanning Annular Eclipse!

(Credit: NASA/A.T. Sinclair).

Live on the west coast of the United States? This weekend, you will get to witness a rare astronomical spectacle, the likes of which the continental U.S. has not seen since 1994.  On May 20th (21st across the International Date Line) an annular solar eclipse graces our fair planet. This eclipse is “annular” in the aspect that the Moon will be only 31 hours from an apogee of 406,450 kilometers around the time it passes between the Earth and the Sun, and thus will appear too small to cover the disk of our nearest star, as it normally does during a total solar eclipse. [Read more...]

AstroEvent: Mercury Down Under.

Mercury to the east as seen from Sydney, Australia on the morning of April 18th.

(Created by the author in Starry Night).

This week’s post goes out to my Astro-Hommies in the southern hemisphere. All too often, astronomy journalism has been accused of being northern hemisphere-centric, US East Coast to be precise. This week, we break with the ranks that are touting the April morning elongation of Mercury as the “worst of 2012, don’t bother” and demonstrating that, depending on your point of view, it may well be the best!

This largely has to do with the current angle of the morning ecliptic before sunrise. For northern hemisphere observers, it is currently very shallow to the horizon. This changes, however, as you venture farther south. It is Fall in Australia, South Africa and South America, and the morning ecliptic is nearly perpendicular to the horizon. I’ve ventured five times south of the equator (not counting a winding mountain road in Ecuador that criss-crossed the equator a dozen plus times!) and felt it bizarre to my northern-based brain to see the Sun transit to the north, or see familiar constellations hanging upside down!

But anyway, this situation bodes well for sighting +0.5 magnitude Mercury this week about a half hour before sunset. In addition, the waning crescent Moon will be about 12° degrees away on the morning of the 18th and just 3 days prior to New. The exact timing of greatest western elongation for the fleeting world is April 16th at 17:00 UTC at 27.5° degrees from the Sun. This is the greatest elongation Mercury achieves in 2012, and very nearly the farthest that it can achieve from the Sun at 27.8°. The reason for this is that Mercury reaches greatest elongation only days after passing aphelion, or its farthest point from the Sun on April 15th at 5:00 UTC. Mercury’s orbit has an eccentricity of about 0.2 or 20%, making its orbit noticeably elliptical and causing evening and morning apparitions of the planet to vary greatly in terms of angular distance.

The tiny 8” arc second disk of Mercury will display a maximum illumination area on April 18th at 2:00 UTC, and is headed from crescent phase to 50% illumination on the 21st. (half phase isn’t precisely on the 18th because Mercury’s orbit is also tilted 7° degrees relative to the ecliptic.

Other mercurial events to watch for are a conjunction with +5.9 magnitude 3.4” arc second Uranus 2.1° degrees apart on April 22nd, 2:00 UTC, and a conjunction next month on May 22nd with Jupiter at 07:00 UTC just 0.4° degrees apart. Yes, this will be very close to the Sun only about 6° degrees away, but note that Jupiter, Mercury, and the Moon will all be within a five degree circle during the annular eclipse only two days earlier! More on this next month… be sure to physically BLOCK the Sun if trying to make this sighting; the eclipsed Sun evening during annularity is brighter than most people realize! And speaking of which, this week is also an excellent time to attempt those daylight Mercury sightings using a similar technique… (OK, we’ve never accomplished this, either!)

Superior conjunction and non-transit of the Sun for Mercury occurs on May 27th. Incidentally, the next transit of Mercury will on May 9th, 2016 and will be visible from North America…. There are 12 transits of Mercury remaining for this century, and curiously, a near “miss” less than 8’ arc minutes from the Sun this year on November 17th… could an active Sun reach out and give us a prominence transit?

Finally, another unique event involves the inner most world on August 15th of this year, when Venus & Mercury reach greatest elongation within 24 hours of each other… more on this to come this summer!