Whether Chaucer’s pilgrims heading to Canterbury, Major League teams flying from grapefruit league venues to hometown stadiums, or mighty Orion nestling westward into evening twilight, we note progressions this month.
Springtime activities are on the uptick and the ground blossoms, enabled by warmth from increasing altitudes of the Sun. Overhead we see evening star fields shifting from bright and beautiful regions centered on Orion that dominated winter evening skies.
Thinner sections of the Galaxy’s envelope of interstellar dust surround its northern pole toward the constellation Coma Berenices. That part of the sky, perpendicular to the band of light that is the Milky Way, is now taking center stage. It provides a clearer window outward beyond stars of the Milky Way toward distant galaxies.
The full moon this month, sometimes called the “Sprouting Grass Moon” or “Full Pink Moon,” occurs April 11. That evening, around 9 pm, the lunar disk may be seen rising high above the southeast horizon in the constellation Virgo.
If the sky is clear I’ll be out making a tour. It will be part visual and part imaginary involving space and time starting with the full moon. The Moon then can serve as the starting point in a series of way stations on a four-dimensional excursion through space and time.
Visually, we easily scan two dimensions—up and down, left and right—starting from the Moon’s disk. For the third dimension, space, and the fourth dimension, time, we need a bit of imagination. We then are enabled to increasingly see past events. For example when we observe the Moon, we experience light that left its surface, 244,000 miles from us, about 1.3 seconds earlier. On the evening of the full moon, April 11, look to the upper right of the lunar disk to spot bright Jupiter. Jupiter is at opposition on April 7, and currently is the closest it will be to Earth this year. These nights Jupiter is about 408 million miles from our eyes and light we see from the solar system’s biggest planet left its surface 37 minutes before its arrival at Earth. We observe Jupiter as it actually looked 37 minutes earlier. To stretch a metaphor about stars, we could see Jupiter intact at 9 pm, even if it somehow had ceased to exist at 8:30 p.m.
Extending connections of distance and time, we go further from home. The bright star Arcturus on the evening of April 11 is visible to the upper left of the full moon, about twice the Moon’s apparent span from Jupiter. Light we now see from Arcturus departed its surface back in 1980. We observe the star this month as it existed that year. To avoid dealing with trillions of miles when describing vast distances in space, it is convenient to say Arcturus is 37 light years from us.
Further distant than Arcturus is the first magnitude star Spica. It forms a triangle with the Full Moon and Jupiter on April 11, below the planet and to the right of the Moon. Spica’s distance from us is about 250 light years. Light we observe when watching this star left its surface around the time President John Quincy Adams was born.
We can extend our distance-time relation to objects seen through the Milky Way’s relatively low dust north polar region. For example, the beautiful globular star cluster known as Messier 3 contains about half a million stars. It’s 33,900 light years from us, within the Milky Way Galaxy. This huge cluster of ancient stars is located about one fist length, seen with your arm extended, to the northwest of Arcturus. Here’s where imagination comes in handy if you’re contemplating, but not using a telescope to see very distant objects.
Onward we go to realms far beyond the fringes of our Galaxy. For example, the enormous elliptical galaxy Messier 87 is at the heart of the Virgo supercluster of galaxies, of which the Milky Way is a member. M87 contains by some estimates 10 trillion stars, more than ten times as many as in the Milky Way. The vast sphere of stars forming M87 is about 54 million light years from us. It’s in the constellation Virgo, at the western corner of an equilateral triangle whose legs about equal the angular span between Spica and Arcturus.
Here’s one more stop on this brief four-dimensional tour of the April evening sky. The Coma Supercluster of galaxies contains more than 3,000 galaxies about 300 million light years from us. It’s the nearest supercluster to the Virgo Supercluster where we live. To point in the direction of the Coma Supercluster, look about half a fist length to the west of Messier 3.
For exciting explorations of the Universe’s dimensions from various perspectives, download the free version of Digital Universe, developed by Brian Abbott and Carter Emmart, at the American Museum of Natural History and the Hayden Planetarium.