Skylights: May 2025

May’s evening sky completes the transition from winter to spring, as the mighty hunter Orion sinks out of view early in the month, A perennial observing challenge, how late into the month can you still see Sirius? If you have a good southwestern horizon, you can probably still see it past the 10th, but can you see it as late as the 15th?

The stars of spring, within the constellations Leo, Virgo, Hydra, and Corvus, present themselves at their best position high in the south during early evening in May. Taking a telescopic tour through the Realm of the Galaxies in Leo, Virgo, and Coma Berenices is no longer a late-night adventure, and the big and little bears, Ursa Major and Ursa Minor, are at their highest positions in the northern sky. 

A week into the month of May, the most prominent of the spring stars, Arcturus, crosses the meridian high in the south at midnight. If T Coronae Borealis, located about 24° to the east of Arcturus in the Northern Crown, were to flare, May would be the ideal time to watch it.

As Capella moves lower in the northwest, notice that Vega is moving higher in the northeast. The two stars appear to be level just after 10:00pm on the 1st, and by mid-month, this position occurs just after twilight. The stars of summer are soon to enter the sky, so enjoy spring while the weather is favorable.

Sun

The Sun enters Taurus on the 14th, after having spent the past 25.5 days traversing Aries.

The year’s first sunset during the 8:00pm hour is on the 16th. Sunsets remain later than 8:00pm until August 4th.

The first day of the year with at least 15 hours of daylight is May 30. Daylight remains longer than 15 hours through August 11th.

Moon

On the 1st, the Moon reaches its most northerly declination of Lunation 1266, at 28.5° north. Note how far north of west it sets, and how late into the evening the crescent Moon remains visible, until almost 1:00am. Due to the libration angle, this is also a good time to look far across the eastern limb of the Moon to view two seldom-seen features, Mare Marginis and Mare Smythii.

On the 2nd, the Moon is 2.3° south of Pollux, in Gemini.

A sight not to miss occurs on the 3rd, when the Moon, Mars and M44 all appear within an area of sky just 3° across. This is ideal for viewing with binoculars, but even better with a small telescope. This is one of those occasions to ponder the scale of our solar system, compared to a relatively nearby object in our galaxy. The Moon is just 1.3 light seconds from us. Mars, at 12 light minutes, is over 500 times farther away. The cluster, at 577 light years, is more distant than Mars by a factor of 25 million. If the distance to M44 from Earth were scaled to the distance between Seagrave Observatory and Ladd Observatory, a distance of about 16 kilometers, the scaled distance between Earth and Mars would be no larger than a tardigrade, or about 0.5mm. 

The Moon is at first quarter, in Cancer, at 9:52am on the 4th. Later that evening, it appears midway between Mars and Regulus. On the 5th, it appears just 2.6° east of Regulus, in Leo.

The Full Flower Moon occurs on the 12th, in Libra. Because the Moon is full at 56 minutes past noon, we get two consecutive nights to enjoy the Moon’s fullness.

On the 11th, the 99.5% illuminated Moon rises at 7:19pm, 35 minutes before sunset. During the evening, look 4.5° north of the Moon to find Zubenelgenubi (alpha Librae), a bright and wide double star with a separation of 331 arcseconds.

The 99.7% illuminated Moon sets at 5:00am on the 12th. The 99.7% rises at 8:23pm, 28 minutes after sunset, and sets the following morning at 5:30am on the 13th, 3 minutes before sunrise, at 99.4% illumination.

The Moon joins Antares (alpha Scorpii) on the 13th, appearing just 0.8° south of the red supergiant. When the Moon passes south of Antares, it demonstrates how far south the Moon is from the ecliptic. It is at its most southerly declination of -28.5° on the 15th.

Last quarter Moon is at 7:59am on the 20th, in Aquarius.

During its waning crescent phase, the Moon joins Saturn and Venus in the morning sky, rising 4.2° west of Saturn on the 22nd, 7.0° west of Venus on the 23rd, and 7.1° northeast of Venus on the 24th.

New Moon, marking the start of Lunation 1266, is at 11:02pm on the 26th.

Mercury

For a brief time on the afternoon of the 12th, both Mercury and Venus will be transiting the non-zodiacal constellation Cetus.

Mercury is visible in the morning sky in early May. During the first week of the month, the innermost planet rises only about 45 minutes before sunrise. 

Mercury spends a brief time (just about 2 hours) on the 12th moving through Cetus. It then moves into Pisces.

Mercury becomes difficult to observe after mid-month due to its low apparent angle on the horizon as it moves towards its superior conjunction on the 30th.

Venus

Venus is visible all month as the “Morning Star,” and telescopically still appears as a waxing crescent, extending from 29.2% illumination on May 1 to 49.1% on the 31st. Its angular diameter diminishes from 36.3 to 24.0 arcseconds.

Venus spends most of the month in Pisces, but spends two days in Cetus, from the 12th to the 13th, briefly sharing the whale constellation with dwarf planet Ceres, about 13° to its south-southwest.

The waning crescent Moon joins Venus on the 23rd and 24th.

By the end of the month, Venus is visible two hours before sunrise, and reaches its greatest elongation, 45.9° west of the Sun, on the 31st.

Mars

Mars remains in our evening sky throughout May, traversing the gulf between both the twin stars of Gemini and Regulus in Leo, which is occupied by the dim stars of Cancer. Early May presents a spectacular viewing opportunity, as the Red Planet moves across the open star cluster M44, the Beehive. It lies within 1° of the cluster from the 2nd through the 6th, appearing within the cluster’s northernmost members on the 4th. The highlight of the passage, however, is on the evening of the 3rd, when the 44.3% waxing crescent Moon, Mars, and M44 all appear within a 3° field. A small telescope will provide the best view.

Mars is now considerably distant, passing 1.5 au on the 9th, but remains at first magnitude, putting it on par with Pollux and Regulus. Mars presents a rather small 6 arcsecond gibbous disk, requiring a large telescope to discern any surface details.

Mars enters Leo on the 25th, and the Moon pairs with Mars again on the 31st, although a few degrees more distant than the one earlier in the month.

Jupiter

At the beginning of the month, Jupiter lies along the line extending from Sirius through Betelgeuse. Moving eastward through Taurus, Jupiter is being chased lower in the sky by increasing later twilight. The planet is 1.0° north of M1, the Crab Nebula, on the 14th.

The 22-hour-old, 1.3% illuminated crescent Moon is 8.9° west-northwest of Jupiter on the 27th, and the following evening, the 5.2% crescent appears almost directly above it, by 7.6°, on the 28th.

Jupiter and its moons remain bright enough to be observed even through twilight and while low in elevation.

Moons of Jupiter

See a triangle of Io, Europa, and Callisto that appears to be pointing towards Jupiter at 9:30pm on the 2nd. 

The 3rd sees a tight grouping of Callisto, Io, and Ganymede to the west of Jupiter, with Europa close by to the east.

Io and Europa are spaced equidistant to the east, and west of Jupiter, respectively, on the 5th, with Ganymede and Callisto farther out on either side.

On the 7th, Ganymede and Io form a perpendicular line on the east side of Jupiter, with Europa close to the planet’s limb on the west, while its shadow remains on the planet’s disk. Callisto is farther out.

Europa and Io form a perpendicular pair to the east of Jupiter on the 9th, with Ganymede and Callisto extended out to the west.

On the 11th, with Io and Europa visible to the east and west of Jupiter, and Ganymede in shadow, Callisto grazes Jupiter’s northern limb.

On the 31st, all four moons are positioned close to their maximum elongation from Jupiter, with Callisto and Europa to the east, Io and Ganymede to the west of the planet.

Saturn

Saturn is visible low in the east before twilight, in Pisces. It is slowly overtaking Venus in elevation, with the two planets being parallel to the horizon on the 6th.

Saturn reaches equinox on the 6th. This is the point at which the subsolar latitude on Saturn is 0°, which is also when the ring plane is illuminated edge-on. Since neither side of the rings are in direct sunlight, the only illumination is primarily from Saturnshine.

As Earth passed the ring plane several weeks ago, we are now looking at a 2.7° angle of incidence onto the southern face of the rings. Saturn is still low in the east before sunrise, but observers using a larger telescope should see the dimly-lit rings, which are casting no discernible shadow on the globe of the planet. 

The waning crescent Moon joins Saturn in the morning sky on the 22rd, appearing 4.2° west of the ringed planet. On the following morning, the Moon appears between Saturn and Venus.

With Saturn’s low ring plane angle, the opportunity exists to watch its moons and their shadows transit the disk of the planet. Many of Saturn’s moons are too small to easily detect without large telescopes and exceptionally good sky conditions, in addition to having Saturn well-placed above the horizon. The moon whose shadow is the easiest to see is Titan, the second largest moon in the solar system. Titan orbits Saturn every 15.9 days, and its shadow has been appearing on Saturn’s cloud tops at this regular interval since last year. Unfortunately for us, the timing of Titan’s shadow transits has had them occurring when Saturn was either below the horizon, or during daylight. Fortunately, we can look forward to beginning to see them in July.

Uranus

Located in Taurus, Uranus is at conjunction on the 17th, and will not be easy to observe until later in June.

During conjunction, Uranus will be occulted by the Sun's disk for approximately 8.5 hours starting at 3:00pm. This isn’t a particularly rare occurrence, but it does only occur when Uranus passes through certain portions of its orbit. Occultations began in 2024 and continue through 2033. There is then a gap of non-occultation conjunctions until 2064.

Neptune

Neptune is low in the east before dawn, in the vicinity of Venus and Saturn. At magnitude 7.8, it isn’t very well placed for observing until later in the month.

Venus is located 3.2° northeast of Neptune on the 7th. On the 9th, draw a line between Venus and Saturn. Neptune lies directly on this line, 4.3° from Venus and just 2.8° from Saturn.

Minor Planets

Asteroid 4 Vesta, located in Libra, is at opposition on the 2nd. The magnitude 5.7 asteroid is 1.181 au from Earth, and can be seen 2.2° northeast of mu Virginis, and 9.7° southwest of the large and bright globular cluster M5. It is moving westward about ¼° per day, and crosses from Libra into Virgo on the 9th.

Also at opposition in May is asteroid 3 Juno. At twice the distance and 1/50th as bright, Juno is located about 15* east of Vesta, in Serpens. Juno can be found about 0.2° north of magnitude 3.5 mu Serpentis on the 5th, and tracks eastward throughout the month, getting no brighter than magnitude 10.0.

2 Pallas, at 10th magnitude, is moving northeastward in eastern Delphinus. By the end of the month, it moves within 1.5° of the globular cluster Caldwell 42 (NGC 7006).

Located a few degrees below Saturn, dwarf planet Ceres remains low in the southeast before sunrise. Prospects for observing the 9th magnitude object improve in June.

Meteors

The eta Aquarid meteor shower is a long-duration shower that is active for nearly a month, beginning in late April, with best prospects for seeing meteors within a few days of the peak on May 3rd-4th. The shower favors southern hemisphere observers, and doesn’t gain much attention in the north due to the radiant being located near the water jar asterism of Aquarius near the peak activity period, and this area of sky is only above the horizon for about two hours before twilight. The waxing crescent Moon will be out of the sky during the morning hours.

The eta Aquarids are one of two significant meteor showers that originated from Comet 1P/Halley, which replenishes the meteor stream every 76 years.