And then there were three

Jupiter has now disappeared from the evening sky and Venus no longer graces the morning twilight. Sadly, this means we have only three naked-eye planets to follow this month, and all appear in the hours before sunrise.

Saturn is the first of this trio to rise, and it also is the farthest from Earth. At the beginning of May, the ringed planet clears the eastern horizon more than four hours before the Sun. From midsouthern latitudes, it climbs some 30° high by the beginning of astronomical twilight. The planet shines at magnitude 1.0 and moves slowly eastward against the backdrop of eastern Aquarius the Water-bearer.

The planet’s high altitude delivers marvelous observing opportunities for those with telescopes. The great views come despite the rings being far less conspicuous than usual. They currently tilt just 3° to our line of sight. Although this makes the dark Cassini

Division that separates the outer A ring from the brighter B ring harder to discern, it opens up other opportunities.

Now is a good time to look for atmospheric details on Saturn’s disk, which measures 17" across the equator at midmonth. This is also prime time for viewing the planet’s moons. Eighth-magnitude Titan — the fifth-brightest moon orbiting another planet, after Jupiter’s four Galilean satellites — is always easy to spot. But all you need is a 10-centimeter or larger instrument to bring in the 10th-magnitude trio of Tethys, Dione, and Rhea.

A nearly Last Quarter Moon occults Saturn on May 31 for observers in the southern third of South America. From

Buenos Aires, Saturn’s disk begins to disappear behind the Moon’s bright limb at 6h42m UT; it takes about 50 seconds for the Moon to complete the job. The rings start to reappear from behind the Moon’s dark limb at 7h30m UT.

Look well below Saturn to pick up the ruddy glow of

Mars. The magnitude 1.1 Red Planet trails Saturn in the morning darkness. It travels eastward through the constellation Pisces the Fish for most of the month but clips the corner of Cetus the Whale during May’s second week.

Mars still lies far from Earth, so it appears only 5" across through a telescope. Although you likely won’t make out much detail, you might catch a brief glimpse of the planet’s south polar cap.

To the lower right of Saturn and Mars lies the last of our morning planets, Mercury.

The innermost planet reaches greatest elongation May 9, when it lies 26° west of the Sun, stands 13° high in the eastnortheast an hour before sunrise, and shines at magnitude 0.5. A telescope shows a disk that spans 8" and appears 40 percent lit. This is Mercury’s finest morning apparition of 2024, so don’t pass up the opportunity to view it.

The starry sky

A year ago, I gave a public address in Mizoram, India, on some of the great contributions women have made to astronomy. I talked about Indian physicist Bibha Chowdhuri (1913–1991), who performed important research on cosmic rays, and mentioned that the International Astronomical Union (IAU) had named a star after her. HD 86081, which lies just south of the celestial equator in the constellation Sextans the Sextant, is now officially called Bibha.

Despite being in the southern sky, the star reaches fairly high in India’s sky because Mizoram lies at a relatively low northern latitude.

Not only is astronomy for everyone, but the sky’s equatorial region is accessible to all. This made me reflect on the fact that although we often talk of northern and southern constellations, the celestial equator passes through many of these star groups (including Sextans). I decided to see how many constellations belong to both hemispheres, and found 15: Pisces, Cetus, Taurus, Eridanus,

Orion, Monoceros, Canis Minor, Hydra, Sextans, Leo, Virgo, Serpens, Ophiuchus, Aquila, and Aquarius.

You may be surprised at some of the members on this list. Most people think Taurus the Bull lies well to the north, but this rather large constellation extends just over 1° south of the equator in the region near 10 Tauri. Another intriguing entry resides just to the east, where the northernmost part of Eridanus the River juts barely north of the equator.

Another “northern” constellation, Leo the Lion, actually has more than 50 square degrees that belong to the southern sky. The region includes the 4th-magnitude stars Upsilon (υ) and Phi (ϕ) Leonis. This part of the sky stands high in the north on

May evenings.

The IAU officially adopted the constellation boundaries in 1928, following the work of Eugene Delporte, and published them in 1930. These were based on the equator and equinox of 1875, when the northern edge of Eridanus and the southern edge of Canis Minor coincided exactly with the celestial equator. The IAU chose the epoch of 1875 because decades before it approved all the borders, Benjamin Gould (1824–1896) already had drawn up borders for the southern constellations based on 1875 coordinates.

However, precession has altered not only the right ascensions and declinations of the stars, but also the coordinates of the constellation boundaries. This means our list must inevitably change. For example, by around the year 2460, all of Taurus will reside in the northern celestial hemisphere.