Star Calendar – May, 2015

Star Calendar

May 2015

Star Calendar Planets:

Moon marks the vernal equinox point as it rises 4-5 AM as a waning crescent on the 14th. On the 20th the Moon crosses the summer colure (but somewhat below the ecliptic) and one can then see/feel how far it is to summer after sunset on that evening. On the 27th the Moon sits just about exactly on the fall equinox after sunset.

Mercury is highest for this evening appearance on the 7th, but is also already getting dimmer. He is stationary to stars, beginning retrograde motion, on the 19th and in only 8 days overtakes Mars in their race toward the Sun. Mercury crosses in front of Sun on the 30th and will make a poor morning appearance in June.

Venus continues to be seen slightly higher and brighter each evening this month. The Moon makes a nice enhancement on the 21st as she approaches a fetching alignment with Pollux and Castor of Gemini – coming in early June.

Sun movement along the ecliptic is slowing down, bringing meridian crossings (solar noons) closer together (shortening the interval of the whole day). At the same time there is an increasing advancement of the Sun’s equatorial-coordinate movement each day (lengthening the whole day interval). The latter factor overtakes the former in magnitude on the 14th of this month – see Equation of Time below. It is odd that the cross-quarter of the seasons (here, the midpoint between vernal equinox and summer solstice, on April 30) just about coincides with the 1/3 point of our Roman calendar year, on May 1.

Mars (unobservably) approaches the Sun with stealth and will creep around and behind in mid-June.

Jupiter stands halfway to the zenith in the WSW after sunset. To the left, and somewhat higher, is Regulus, the heart of Leo. The pair will slowly converge and appear lower each evening – their accomplished meeting in August then being obscured by proximity to the Sun.

Saturn is in opposition and up all night, rising below the much higher Arcturus in the evening. Saturn, now at maximum brightness, just about reaches “zero magnitude”, which is defined as the brightness of the star Vega (to be seen further to the NE these evenings). Vega is the brightest star of our summer sky.

The Equation of Time is the difference between the actual movement of the Sun across our sky and an averaged (mean) or regularized version used in modern time-keeping. The actual movement of the Sun accelerates and decelerates on two cycles: Once per year due to orbital eccentricity (making the noon-to-noon, meridian-crossing days longer in northern winter) and twice per year due to the tilt of the ecliptic (the equatorial “x-component” of the Sun’s sinusoidal movement is larger at solstices than at equinoxes). The sum of these two cycles yields the Equation of Time used to reconcile sundials to mechanical clock-time. A smaller maximum occurs in the middle of this month, a “zero” in mid-June.

Star Calendar Days:

1      Sunrise/sunset in Spring Valley at 5:54/19:53 EDT (13h59m daylight)

          1/3 of calendar year A.D. 2015 has elapsed; more precisely at 6:56:24PM

          Moon rising above Spica in evening, SE at 9PM

3      Full Flower Moon 11:42PM – between Spica and Saturn

4-6     Moon near Saturn – rising late evening

7      Mercury at greatest evening elongation, WNW at 9-9:30PM

10      Venus at highest declination (north of c.equator), 26 degrees N.

14      True Sun (shadow time) is 3m41s ahead of Mean Time (clocks); then reducing

17-18 Moon is New at midnight

19      Mercury stationary to retrograde against stars

21      Moon setting beside Venus, WNW 9-11PM

22      Saturn in opposition; nearest and brightest (similar to Vega or Arcturus)

23      Moon below Jupiter, WSW at 9PM

27      Mercury overtakes Mars in their race toward the Sun (not visible)

30      Mercury crosses nearside of Sun (inferior conjunction)

31      Sunrise/sunset in Spring Valley at 5:26/20:22 EDT (14h56m daylight)

About pbdavis

Paul Davis is a former resident of the Threefold Community. He has been a teacher of Celestial Navigation, a Planetarium lecturer, and offered evening Astronomy classes at Sunbridge some years ago. He is now living in New Hampshire.
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