April, the fourth month, is named from the Latin word “aperire,” which means to open, as in a flower bud. The poet Thomas Stearns Eliot called April “the cruelest month” because it holds out the promise of summer while last year’s dead vegetation lies sodden underfoot drenched in cold spring rains.

Focus on the planets

All five of the naked-eye planets will be visible on the western horizon during the evening hours in April. When Mercury makes its appearance shortly after midmonth, viewers will be treated to the beginnings of the most spectacular planetary alignment to have taken place in the last 20 years.

Mercury comes into view low in the west at midmonth and, by April 30, is visible for an hour and a half after sunset.

Venus is the lowest, but the brightest, of the four remaining naked-eye planets. Look for Venus shining through the glow of the setting sun in the west-northwest at dusk.

Mars, the dimmest of the naked-eye planets, is situated to the upper left of Venus sandwiched between it and Saturn for much of the month. On April 22, Mars is directly to the right of Aldebaran, giving viewers the chance to compare their reddish hues.

Jupiter is very high in the west at sunset and is the brightest of the naked-eye planets after Venus. The two stars above Jupiter are Castor and Pollux, the “twins” of Gemini.

Saturn lies to the lower right of Jupiter in close proximity to Aldebaran. The ring system of Saturn still maintains a favorable open tilt for viewing.

Uranus, Neptune and Pluto are just starting to emerge into predawn view but, with the great show taking place during the evening hours, are hardly worth the trouble.

Our celestial neighborhood

Now that we’ve had a look at Earth’s immediate neighborhood of the solar system, let’s turn our attention to the solar system’s neighborhood, the Milky Way galaxy. The Oort cloud of comets lies about 100,000 astronomical units from the sun. An AU is the distance from the Earth to the sun or 93 million miles.

If we consider the Oort cloud to be the limit of the solar system, it is a very large neighborhood indeed but it pales in comparison to the Milky Way galaxy. Distances in the Milky Way are measured in light-years, the distance light travels in one year. It figures out to be about 6 million million miles!

The Milky Way is roughly 100,000 LY in diameter, somewhere between 5,000 LY and 15,000 LY in thickness, and contains about 200 billion stars. The sun, a rather insignificant stellar specimen, lies in an arm of the galaxy about 28,000 LY from the center.

The Milky Way can be thought of as shaped something like a discus with arms like a pinwheel. British scientist J.B.S. Haldane once remarked, “The universe is not only queerer than we imagine, but queerer than we can imagine.”

As we look at some of the strange inhabitants of the Milky Way galaxy in future Maine Skies columns, we will see how right he was.

April events

1 Sunrise, 5:17 a.m.; sunset, 6:02 p.m.

4 Moon in last quarter, 10:30 a.m.

7 Spring forward! This is the first Sunday in April, so set your clocks ahead one hour as the nation goes on daylight-saving time.

10 The moon is at apogee, or farthest distance from the Earth, tonight.

12 New moon, 3:22 p.m.

15 Taxes are due today. The pain can be lessened by enjoying the celestial show on the western horizon about an hour after sunset. Starting with Venus at the bottom, a diagonal line to the upper left passes through Mars, Saturn and Jupiter in that order. The crescent moon is on one side of Mars and the Pleiades on the other.

19 The sun enters Aries on the ecliptic.

20 Moon in first quarter, 8:49 a.m. The sun enters the astrological sign of Taurus but, astronomically, has only just entered Aries.

22 This is the peak night for the Lyrid meteor shower. They will radiate from a region in the vicinity of the bright star Vega in the constellation Lyra. A normal rate is about 15 meteors per hour but this year they will be obscured in part by a bright moon.

25 The moon is at perigee, or closest approach to the Earth, tonight.

27 Full moon. The full moon of April is called the Grass Moon or Egg Moon.

30 Sunrise, 5:27 a.m.; sunset, 7:39 p.m. Check out the show on the western horizon again tonight. Mercury, Venus and Saturn form a diagonal line starting from the horizon and going to the upper left. Aldebaran is to the right of Venus and Mars is nestled just to the lower right of Saturn. Jupiter sits in solitary splendor well above, forming its own triangular grouping with Castor and Pollux.

Clair Wood taught physics and chemistry for more than a decade at Eastern Maine Technical College in Bangor.