What do astronomers mean when they say they can look back into time? The further a star is from Earth, the longer it takes for light from it to reach us. Light travels at 186,000 miles per second. In a year it travels a ‘light-year,’ which is approximately 6 trillion miles. Stars, galaxies, and other celestial objects lie so far from Earth that light takes many years to travel across space to Earth and the distance to them is given in light-years.

The closest star to Earth – beyond our own sun – is Alpha Centauri, about four light-years away. The light we see tonight from Alpha Centauri actually left more than four years ago. In other words, we are not seeing what is taking place on the star right now but how it appeared in 2002. Other objects lie at much greater distances, and the light astronomers study actually left the object many years ago. This is what is meant by looking back into time.

Light from Betelgeuse in Orion left around 1420 when the Black Plague ravaged Europe, and that from Polaris, the pole star, in 1540 as the Protestant Reformation spread across Europe. And these are some of the closest neighbors to us. The most distant object visible to the naked eye is the Great Andromeda galaxy that lies about two million light-years away. The light you see from this left long before there were humans around to see it. And the most distant object thus far seen is a galaxy estimated to be 13.25 billion light-years away. The light astronomers used to make the measurement has been traveling for nearly as long as the universe is old and longer than the Solar System, and Earth have been around to receive it.

Focus on the planets

A visitor to the night sky, comet Schwassmann-Wachmann-3, should be easily visible through binoculars, and possibly even with the naked eye, as it leaves the vicinity of Arcturus, sweeps through the keystone of Hercules, passes Vega and travels all the way around the sky to Cetus near Venus during May.

Mercury appears briefly above the eastern horizon at dawn as May opens and then disappears from view until nearly the end of the month. Look for Mercury very low in the west-northwest about 45 minutes after sunset the last few days of May.

Venus shines brilliantly low in the east about half an hour before sunrise and maintains the same altitude as it moves through Pisces.

Mars opens the month high in the west around 10 p.m. with the crescent moon to the lower right, The two stars immediately above Mars are Castor and Pollux of Gemini. By month’s end, Mars will have traveled to the immediate vicinity of Saturn.

Jupiter is on the southeastern horizon around 10:00 p.m. and remains in the sky nearly all night long. The nearly full moon is situated to the immediate upper right of Jupiter on May 11. Surface features such as Jupiter’s equatorial bands and storm spots as well as its four major moons are readily visible by telescope.

Saturn opens the month to the upper left of, and is much brighter than, Mars. Note the waxing crescent moon as it passes Saturn on May 3. Saturn’s ring system is tilted to show fine detail and several of the ringed planets moons will be in view for viewers with telescopes.

Uranus lies above the east-southeastern horizon at 5:00 a.m. where it may be spotted as a greenish disk.

Neptune is a small gray-blue disk nestled among the stars of Capricornus about 4 a.m. A star chart will likely be needed to spot these dim, far-distant planets.

May events

1 Sunrise, 5:26 a.m.; sunset, 7:40 p.m. May Day or Beltane celebrated as the cross-quarter day marking the midpoint between the spring equinox and summer solstice. Mars lies to the upper left of the crescent moon around 10 p.m. The two bright stars above Mars are Castor and Pollux while the reddish one hovering above the horizon is Betelgeuse.

4 The moon is just northeast of the Beehive cluster in the evening sky.

5 The peak night for observing the Eta Aquarid meteor shower, which is made up of debris from the Halley comet. Look low to the east-southeast during the predawn hours where a dozen or so meteors per hour should be radiating out of Aquarius.

7 The moon is at apogee, or furthest distance from the Earth today.

11 Look for Jupiter and the nearly full moon shining together in the southeast at dusk.

13 Full moon, 2:52 a.m. The full moon of May is called the Flower Moon, Milk Moon, or Planting Moon.

14 The sun enters Taurus on the ecliptic.

20 Moon in last quarter, 5:20 a.m.

21 The sun enters the astrological sign of Gemini but, astronomically, is still in Taurus.

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

24 Look low in the east at dawn for the crescent moon and Venus together.

27 New moon, 1:27 a.m.

29 The moon, Mars, and Saturn, in that order, form a diagonal line from lower right to upper left in the west at dusk. Look just above the horizon to the lower right of the moon and you might spot Mercury. The two stars to the immediate right of Mars are Castor and Pollux.

31 Sunrise, 4:53 a.m.; sunset, 8:13 p.m.

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