Jupiter reaches conjunction with the Sun on May 13, and is effectively invisible this month.

Venus begins the month of May at a dazzling magnitude of -4.7; this is close to its maximum brilliance for this evening apparition. For months, the planet has been getting larger in angular diameter as measured from "cusp to cusp;" at the same time, its phase has been narrowing. On May 1, for example, its disk is 37 arc-seconds across and 27% illuminated; by month's end, it is a narrow crescent 57" across and only 1% illuminated!

As an interesting aside, such an angular diameter is approaching the roughly 1 arc-minute (60") theoretical limit of resolution of the human eye; is it possible to see the crescent of Venus without optical aid? While it may be possible to make out such a tiny shape in principle, it is likely that such a thin sliver would be impossible to see - especially when we take into consideration that the planet would necessarily appear against a bright twilit (or daytime) sky. Nevertheless, there are numerous anecdotal reports of just this. Can you see the phase of Venus with the unaided eye?

Maximum brilliance is reached when Venus' increasing angular diameter and its narrowing phase combine to result in its "greatest illuminated extent" - its largest area of sunlit surface as measured in the curious units of "square arc-seconds." It dims slowly to magnitude -4.1 by month's end. But the most dramatic change in Venus this month is its dramatic plunge toward the horizon. On May 1, the planet is 35° above the horizon at sunset and sets three and a half hours after the Sun; by the 31st, it is a mere 7° above the horizon at sunset and sets just 45 minutes later! There is a reason for its hurry; next month, it has a very special appointment to keep: a transit across the face of the Sun. (More on that in the June Sky Report.)

Mars appears high overhead shortly after sunset on May evenings. It dims slightly from magnitude 0.0 to +0.5 during the month, remaining brilliant enough to outshine Regulus, the brightest star in Leo. The main problem for observers is that the planet's apparent diameter shrinks from an already-small 10" to less than 8" in May. Such a tiny disk reveals few surface details.

Saturn, just past opposition, is conveniently placed for evening viewing. At midmonth, it shines at magnitude +0.4; it thus slightly outshines Spica which, at magnitude +1.04, is the brightest star in the host constellation of Virgo. The star and planet retain an almost constant separation of 5° all month. A telescope shows the planet's globe to be about 18.7" across on the 15th; the globe, though, is not the first thing you notice! The rings are now 42.5" across, and tilted 14° to our line of sight. (The minimum - when the rings were edge-on - occurred in 2009; the maximum tilt of 27° will occur in 2017.) While you're at it, be sure to check out the moons. Titan, at magnitude 8.4, is the only moon in the Solar System with a substantial atmosphere; it seems to have its own analog of our "hydrological" cycle - but with liquid methane playing the role that water serves on our planet.

Neptune rises about four hours before sunrise by end of May. The 7.9-magnitude planet lies in Aquarius. A larger telescope shows a blue-white disk 2.3" in diameter.

Uranus rises about two hours before the Sun at mid-May. On the 11th, it crosses the border from Pisces into Cetus. In a telescope, the 5.9-magnitude planet appears as a blue-grey disk 3.4" across.

Mercury is in the morning sky, but it is poorly visible this month. It goes through superior conjunction - passing "behind" the Sun - on the 27th, and moves into the evening sky.

Dwarf Planets/Asteroids:

Pluto, Pluto, in NW Sagittarius, is best seen between midnight and dawn; finding it requires a dark sky, a large instrument, and good star-hopping skills.