What do astronomers mean when they say they are looking back into time? Let’s first look at an example here on Earth. Suppose you took a picture in a room filled with people on your cell phone. You then get in your car and drive 100 miles home at a constant 50 miles per hour, a journey that takes two hours. You then show the picture to a friend.

Are you showing them the room as it is now?

Of course not, it is a picture as it was two hours ago. You are looking back into time with respect to the room. There is a speed limit in the universe. The speed of light is a constant 186,000 miles per second. At that speed it takes about eight minutes for light to reach us from the sun and 2.1 seconds from the moon.

But objects in the universe are so distant that, even at the speed of light, the time it takes for light to reach us is a huge number. Thus, astronomers use the light-year, which is equal to the distance light travels in a year or about 6 trillion miles.

The closest star to Earth is Alpha Centauri at 4.3 light-years and the most distant galaxy so far sighted is at 12.8 billion light-years. In 1893, the World’s Fair was held in Chicago and again in 1933. Someone realized the star Arcturus is almost exactly 40 light-years from Earth and light gathered from the star by telescope was focused on a bank of photocells that generated enough electricity to trip a switch turning on the lights at the 1933 opening ceremony. Thus, light leaving Arcturus during the 1893 World’s Fair was used to open the fair in 1933, 40 years later.

Focus on the Planets

Mercury lies in the west about an hour after sunset and makes its best appearance for 2015. Initially bright and fairly high above the horizon, it fades and sinks as the days pass, disappearing about mid-month.

Venus blazes high on the western horizon where it will remain all month as the evening twilight deepens. It cannot be mistaken for any other celestial object.

Mars is lost to view on the far side of the sun during May.

Jupiter rides high in the west-southwest during the early evening and sets around midnight. After Venus, it is the brightest planet in the sky. Telescopes will reveal the planets dark equatorial bands and the continuous dance of its four moons as the change position from night to night.

Saturn comes up in the southeast around 9:30 p.m. as May opens and remains up virtually all night setting in the southwest at dawn. The pronounced tilt of the ring system gives an excellent view of the dark Cassini Division separating the A-ring from the brighter B-ring. Saturn’s large moon Titan also can be easily spotted.

Uranus rises just above the eastern horizon as dawn breaks and will be difficult to spot.

Neptune rises around 3:00 a.m. and is low in the southeast where good optical aids will be needed to spot it.

May Events

1: Sunrise, 5:26 a.m.; sunset, 7:40 p.m. This is May Day, or Beltane, a cross-quarter day marking the midpoint between the spring equinox and summer solstice.

2: Look to the west about an hour after sunset to see Venus shining brightly with Mercury far to its lower right.

3: Full moon, 11:00 p.m. The full moon of May is the Flower Moon, the Milk Moon, or the Corn Planting Moon. Note that golden Saturn is just to the lower left of the moon during the late evening.

6: This is the peak night for the Eta Aquarid shower, however, the waning gibbous moon will drown out the fainter ones, so expect around 10 sightings an hour out of the east-southeast.

11: Last quarter moon, 6:36 p.m.

14: The sun enters Taurus on the ecliptic.

15: The moon is at perigee or its closest approach to Earth.

18: New moon, 12:13 a.m.

21: The sun enters the astrological sign of Gemini but astronomically is still in Taurus. Venus sparkles to the upper right of the moon tonight with Jupiter far to the upper left.

25: First quarter moon, 1:19 p.m.

26: The moon is at apogee or its farthest approach to Earth.

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

Send astronomical queries to Clair Wood at cgmewood@aol.com or care of the Bangor Daily News, Features Desk, P.O. Box 1329, Bangor, ME 04402.