Skylights: April 2025

The skies of April bring us warmer, yet shorter nights. At no other time of year do the seasonal constellations appear to change as quickly as they do now. Early in the month, we’re still greeted by the Winter Hexagon and its component constellations soon after twilight fades. We can take one last look at some of our favorite sites in Taurus, Orion, and the hunting dogs Canis Major and Canis Minor. By month’s end, when twilight ends, three-quarters of an hour later into the night, the Pleiades and Orion’s Belt are barely above the horizon, setting simultaneously into their seasonal hiatus.

To the east, we have our beacon of the season, Arcturus, and high in the southeast are Leo and Hydra.

Galaxy season is upon us, as the north galactic pole, located in Coma Berenices, beneath the handle of the Big Dipper, passes high overhead at midnight, giving us a clear window into the extragalactic universe that is relatively unimpeded by obscuring stars, nebulae, and dust clouds of the Milky Way.

April is also the best time of the year for an observing challenge to spot the largest and brightest globular cluster in the sky, Caldwell 80 (NGC 5139), Omega Centauri. This magnitude 3.7 cluster should be bright enough to be easily visible with binoculars, but because it culminates at barely at a single degree above the southern horizon from our latitude of 41.85°N, the light from its half-billion stars is highly extincted by about 8.5 magnitudes through 30 times as much airmass than it would be if it were shining at the zenith. As such, you’ll need a large telescope, a vantage point that affords an unobstructed view to the true horizon south, and a very transparent sky, free of any haze or fog.

The Blaze Star, T Coronae Borealis, is still at its 10th magnitude quiescent state. However, astronomers at Thuringian State Observatory in Germany recently detected significant changes to the spectral emission lines of T CrB, suggesting that its accretion rate may be accelerating, possibly suggesting that the nova is imminent. T CrB, located in the Northern Crown constellation, is well-placed for observing throughout April. Although it is low in the northeast in early evenings, it quickly rises and is visible all night. When the star does go nova, it is expected to be as bright as Alphecca (alpha Coronae Borealis), 4.5° to its east.

Sun

As the days (and nights) are getting warmer, you probably wouldn’t give much thought to the fact that Earth is currently moving away from the Sun in its orbit. At about 2:00am EDT on the 3rd, we’re about halfway between perihelion and aphelion, and we cross the 1 au mark. We will remain more distant than 1 au from the Sun until October 5.

The Sun crosses declination +12° 43’ 09” on the 23rd, marking the halfway point in the Sun’s northerly trek between equinox and solstice. The Sun remains north of this line until August 19.

And with the Sun’s northward progression, we rapidly gain additional hours of daylight in April, with the first 13-hour daylight period occurring on the 7th, followed by the first 14-hour daylight period on the 30th (for observers near latitude 41.84° north). The length of daylight will remain greater than 14 hours until August 11, and greater than 13 hours until September 3.

The equation of time is 0 on the 15th. The equation of time is the measure of the discrepancy between mean solar time and apparent solar time, an effect caused by the eccentricity of Earth’s orbit.

One sidereal rotation of Earth takes 23 hours, 56 minutes and 4.1 seconds. This is, observing the stars, let’s say Spica for example, the interval between successive transits of the meridian of Spica. Because Earth has moved around its orbit by approximately 1/365th of a revolution during this interval, an additional four minutes elapses before the Sun returns to the meridian on successive days. This is the mean solar time. 

However, due to the elliptical orbit, and because Earth moves faster when it is closer to the Sun, and slower when it is farther away, this time difference varies. When Earth is closer to the Sun in January, it takes additional time for the Sun to return to the meridian, and when Earth is farther away, in July, less rotation is needed to make up the difference. Drawn on a graph over time, this discrepancy forms a double sinusoidal wave, which has a shorter amplitude in the spring, and a larger one in the autumn. When this graph is drawn on the axis of the declination of the Sun, it forms an asymmetric “figure-eight” pattern, with the smaller lobe to the north, and larger lobe to the south. You may have seen this graph, known as the analemma, as a composite photo of the position of the Sun over the course of a year. The meridian axis bisecting the graph is the zero reference, which the Sun appears to cross four times per year, one of them being April 15th.

After spending 37 days in Pisces, the Sun enters Aries on the 18th.

Our first sunrise before 6:00am occurs on the 19th. The Sun continues to rise earlier than 6:00am until August 21.

Moon

One of the most beautiful sights that can be seen in the evening sky is when the crescent Moon appears near the Pleiades cluster. Such a pairing occurs early in the evening of the 1st, when the 18.0%-illuminated crescent lies just 1.4° above the cluster.

The Moon attains its most northerly position for the month on the 3rd, and for about four hours it passes through the non-zodiacal constellation Auriga. The still crescent Moon sets at 2:09am on the 4th.

The first quarter Moon occurs at 10:14pm on the 4th, in Gemini.

The following evening, the 5th, the slightly gibbous Moon is located near the twin stars Pollux and Castor in Gemini, as well as 2.6° from Mars.

On the 6th, the Moon is 4.5° northeast of the Beehive Cluster, M44, in Cancer, and on the 7th-8th, it lies near Regulus, in Leo.

The Moon is full at 8:22pm EDT on the 12th. This is the first full Moon of the spring season, and is referred to as the Pink Moon. The Moon rises just an hour before it becomes full, and just as the Sun is setting, which makes for the most photogenic full Moon rises, as the foreground landscape will still be brightly illuminated. Keep watching the Moon as it rises and the sky undergoes the vibrant Belt of Venus phenomenon, when the edge of Earth’s shadow creeps  upward from the eastern horizon to overtake the remaining blue, violet, and pink hues from the fading twilight. As the sky continues to darken, you may notice a bright star just 0.5° north-northwest of the Moon, that’s Spica, the brightest star in Virgo.

The Moon sets at 6:09am on the following morning, the 13th, just as the Sun is rising.

On the 16th, the 90% illuminated Moon occults Fang (pi Scorpii), the magnitude 2.9 star marking the southern claw of the scorpion, beginning at 5:11am and reemerging after sunrise. Later that evening, the waning gibbous Moon appears 3.1° east-southeast of Antares.

The Moon is last quarter in Sagittarius at 9:36pm on the 20th.

On the morning of the 25th, the 8.6% illuminated waning crescent Moon, Venus, and Saturn form a 4.5° triangle, making the grouping visible in the same binocular field. For an additional challenge, use a telescope to find Neptune just 0.5° south of the Moon.

The Moon is new at 3:31pm on the 27th, beginning Lunation 1265. A young crescent sighting opportunity opens 29 hours later, when the 2.2% illuminated Moon can be seen after sunset. Also on this evening, the Moon is just 4.0° west-southwest of the Pleiades. You may also find Uranus 3.9° south of the Moon.

The Moon appears 11.1° west-northwest, and 6.0° northeast of Jupiter on the 29th and 30th, respectively.

Mercury

Mercury is low in the eastern sky before sunrise in early April. Through a telescope you can see Mercury going through its waxing crescent phase, though you’ll need rather high magnification and a very clear horizon to see it.

Mercury is at greatest elongation in the morning sky on the 21st. Although it reaches a generous separation of 27.4° west of the Sun, this is not a very favorable viewing opportunity due to its low angle on the ecliptic during April mornings, which allows Mercury to be seen for less than an hour before sunrise.

Venus

Having passed its inferior conjunction last month, Venus is in the eastern sky before sunrise, where it will remain in its position as the Morning Star for the remainder of the year.

The brilliant planet is up an hour before sunrise on the 1st, putting it in a good position to observe its remarkably thin crescent. 

Starting from the 15th, Venus is visible for at least 90 minutes before sunrise.

Venus is at its brightest on the 22nd, shining at magnitude -4.8 at a phase illumination of 22.1% on its 41.5 arcsecond disk. 

Venus tracks just a few degrees northwest of Saturn throughout April, as the two planets slowly draw closer. The waning crescent Moon joins Venus and Saturn in a 4.5° celestial triangle that will be a fine sight in binoculars on the 25th. On the 29th, the two planets will be at their closest, 3.8°.

Mars

Although Mars doesn’t reach its quadrature (90° elongation from the Sun) until the 20th, you’ll notice it positioned close to the meridian during twilight early in the month. This coincides with its highest elevation in the south, and it will continue to appear lower in the sky each evening, but we still have plenty of time to observe Mars in our evening sky..

Mars crosses the imaginary line connecting Pollux in Gemini and Procyon in Canis Minor on the evening of the 1st, subsequently “exiting” the Winter Hexagon asterism. This is a good time to take note of its eastward progression each evening. 

As Mars is becoming more distant, it also continues to fade. Now at magnitude 0.5, it is of similar brightness to the nearby stars Betelgeuse and Procyon. Although Mars is now over 1 au away, its 8.2 arcsecond globe will still show surface detail in larger telescopes and during nights of favorable seeing conditions.

The waxing gibbous Moon joins Mars on the 5th, when the pair will be separated by just 2.6°.

Mars lies in line with Castor and Pollux, in Gemini, on the evening of the 10th.

When we think of planets going through phases, the inner planets Venus and Mercury are the ones that come to mind, both of which are currently displaying crescents.

The outer planets also undergo phases, but because they never come close to passing between the Sun and Earth, they show only either gibbous or full phases. The full phases occur at the point of opposition, and at all other times they appear as gibbous. The extent of the gibbous phase of the more distant planets is detectable, yet subtle. Jupiter’s darkened limb can be seen through large telescopes or images, Saturn and its rings provide shadow cues pointing to its darker limb. Mars, given its proximity to Earth compared to the more distant outer planets, can display an obvious gibbous phase near its points of quadrature (90° elongation from the Sun). One of these times is occurring now, and on the 10th, Mars reaches its minimum illuminated fraction, 89.9%. This is approximately the same illumination of the Moon when it is about 3.5 days before full.

Mars reaches its point of eastern quadrature (90° from the Sun) in Cancer on the 20th. Through a telescope you will see a distinct gibbous phase, but its 7.3 arcsecond disk may not reveal much in the way of surface markings unless you observe it with a larger telescope.

The last week of April sees Mars within a binocular field of view of the Beehive Cluster, M44, in Cancer. The Red Planet closes in on the cluster by about 0.4° eastward each evening until May 4, when it lies just on the northern edge of the cluster.

By the end of the month, Mars shows a disk smaller than 7 arcseconds, making resolving surface details difficult without a large telescope and exceptionally good seeing conditions.

Jupiter

Jupiter is quickly losing its position of prominence in the sky during April. It still resides in Taurus, but it and its host constellation are moving lower into the western sky.

The waxing crescent Moon pairs with the giant planet on the 2nd-3rd. Around 10:00pm, they will be separated by less than 5°.

On the 10th, Jupiter will be just 1.0° from the open cluster NGC 1746, whose brightest members are 7th magnitude. The pairing should be appreciated in larger telescopes with lower magnification, wide-field eyepieces..

Jupiter crosses the line connecting Capella in Auriga to Rigel in Orion on the 24th.

Moons of Jupiter

A very close pairing of Europa and Ganymede on the east side of Jupiter can be seen early on the 1st. Just as twilight is fading, they will be within 5 arcseconds of each other.

The moons are arranged in order of orbital radius, extending to the west of the planet, early on the 3rd. Io and Europa will be occulted by Jupiter at 9:18pm and 10:24pm, respectively.

Ganymede goes into occultation at 7:50pm on the 6th, and reemerges at 10:26pm.

Europa and Io make a close pair, just 4 arcseconds, at 10:15pm on the 7th.

On the 8th-9th, to the east of Jupiter, Callisto, Europa and Ganymede form a tight grouping which transforms from a line to a small triangle. Io appears alone on the west side of the planet.

On the 11th, Io emerges from transit at 10:48pm, and its shadow exits the planet’s disk 68 minutes later.

On the 12th, Io emerges from eclipse at 9:06pm, and Europa emerges from transit at 10:18pm.

The moons form two distinct pairs to the east of Jupiter on the 14th: Europa and Io as the innermost pair, and Ganymede and Callisto the outermost.

Another grouping similar to that seen on the 8th-9th is visible on the 15th.

All four moons appear in order of their orbital radius to the west of Jupiter on the 17th.

Io enters transit at 10:34pm on the 17th.

Europa enters transit at 10:28pm on the 19th, and Io emerges from Jupiter’s shadow 34 minutes later.

On the 21st, Europa exits Jupiter’s shadow at 10:02pm.

Ganymede exits transit at 9:28pm on the 23rd. Its shadow begins to cross Jupiter’s disk at 10:36pm.

Io passes behind Jupiter at 9:48pm on the 26th. On the 27th, Io emerges from transit at 9:20pm, followed by its shadow at 10:16pm.

On the 28th, Europa goes into occultation at 8:12pm.

Saturn

If you are able to locate Mercury in the morning sky in early April, you may also be able to find Saturn, and given the still narrow ring angle, it’s worth the effort to try to find it.

The effort will require a telescope, as well as an exceptionally clear east-southeastern horizon. Once Mercury is found, move to the 4 o’clock position away from Mercury, which is moving towards Saturn each morning during the first few days of April. The distance to move decreases as the week progresses, from 4.6° on the 1st, to just 2.0° on the 10th.

On the 1st, the ring plane angle is just -0.8. The value is negative because we’re now seeing the southern face. Saturn’s equinox is still a month away, so the sunlit side of the rings is still the north side, meaning the rings will be rather dark, and very difficult for us to see, especially considering we’re looking through bright twilight at this time.

By mid-month, Saturn rises an hour before sunrise and can be located just below Venus in the morning twilight. Venus and Saturn gradually draw closer as the month progresses. On the 16th, it is 6.1° to the southeast of Venus, and 4.5° on the 22nd. 

Saturn crosses from Aquarius into Pisces on the 19th.

The waning crescent Moon, Venus and Saturn form a 4.5° triangle on the 25th, and Venus is 3.7° north of Saturn on the 27th.

By month’s end, the ring plane angle will increase to -2.5°.

Uranus

Uranus is in Taurus, just a few degrees southwest of the Pleiades. Early April is the last chance to get a good view of the planet before it is lost to evening twilight. In early April, it is 6.7° from the Pleiades, and by month’s end, it is just 5.9° away.

Neptune

Neptune is too low in the east before sunrise to observe until later in the month.

Mercury is 0.7° southeast of the distant ice giant on the 17th. 

On the 30th, Venus, Saturn and Neptune form a triangle with segments 3.8°, 3.3° and 2.8°. Neptune’s magnitude is 7.8, making it a challenge to observe in twilight.

Minor Planets

The distant dwarf planet Haumea reaches opposition on the 22nd. At a distance of 49.001 au, light from the 1900-kilometer-wide egg-shaped world takes 6.8 hours to reach us, glowing at magnitude 17.5, which is detectable with a modest-sized telescope equipped with an astronomical imaging camera. Haumea is a fascinating object: its day is just 3.9 hours long, and it possesses a ring and two known moons. Find Haumea about 1.0° northwest of zeta Bootis.

Asteroid 4 Vesta is well-placed for observing in April. Moving retrograde through northern Libra, it is high over the southeastern horizon by midnight, and its magnitude 6.2 brightness puts it within easy range of binoculars and small telescopes even during nights that are brightly moonlit.

In early April, Vesta can be found 3.1° north of Zubeneschamali (beta Librae). You can also look about 8° south of the large and bright globular cluster M5. Vesta is easy to locate with binoculars at magnitude 5.9 when it passes 3.7° north of delta Librae on the 20th. As the month progresses, it moves in a more west-northwesterly direction towards magnitude 4.5 16 Librae, which it passes within 0.2° south-southeast, and 0.3° southwest of on the 26th and 27th, respectively.

Asteroid 3 Juno is following a nearly parallel track about 14° east of Vesta throughout 2025. The smaller and more distant asteroid requires a telescope to see its 10th magnitude glow. 

2 Pallas swings in a northeasterly direction through Delphinus during April. Although this section of sky is low in the east-southeast just before dawn early in the month, it attains altitude and puts the magnitude 10.4 asteroid in better viewing position later in the month. On the 1st, it is 1.8° north of the globular cluster Caldwell 47 (NGC 6934), and on the 8th, it is 0.3° south-southeast of magnitude 5.2 kappa Delphini.

As the dwarf planet Ceres lies over 8° south of the ecliptic at this time when the angle of ecliptic is low on the horizon before sunrise, the 10th magnitude object is not in a good position to observe this month.

Pluto is in Capricornus and is just high enough in the southeast to begin observing before twilight later in the month. It is 8.1° south of beta Capricorni.

Meteors

The annual Lyrid meteor shower is active during the third week of April, with peak activity occurring on the night of the 21st-22nd. Observers under a dark sky could experience up to 18 meteors per hour, originating from the patch of sky just west of Vega. The 36% illuminated waning crescent Moon rises at 3:19am.

The Lyrids are produced by grains left by comet C/1861 G1 Thatcher, a long-period comet that is not expected to return for another 250 years. 

The eta Aquarid meteor shower is a long-duration meteor 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 originates from comet 1P/Halley, which replenishes the meteor stream every 76 years.