Summer is arriving ... almost

I am always glad to see summer arriving, almost. The outside temperatures are getting better for staying outside longer for more time at the telescope, but the nights are growing shorter so there is less time for viewing.

As we approach the Summer Solstice (June 21 at 4:51 am MDT) the amount of dark grows shorter and the amount of daylight grows longer, for example, on the day of the Summer Solstice there will be about 15 hours of daylight. That means there is only nine hours for dark.

But, that dark is not nine hours long, we have something called "twilight" to contend with. Twilight is that not quite light, but not quite dark after sunset or before sunrise that does not permit good viewing at the telescope.

Plus there is more than one kind of twilight. First up is "civil twilight."

Civil twilight is defined as the time when the geometric center of the Sun is six degrees below the horizon, or roughly 20 to 30 minutes before sunrise or after sunset. That time when it is not quite light, but it is not quite dark either. That is the time when we need to turn our car's headlights on for example.

Next is Nautical Twilight which is when the geometric center of the Sun is 12 degrees below the horizon. Generally used to define when the horizon at sea can still be seen and measurements using some of the brighter stars can still be taken but artificial lighting is needed to see objects around you.

Last is the kicker, at least for us astronomers. Astronomical twilight is when the center of the Sun is 18 degrees below the horizon. This is the period when it is truly dark and astronomers can do their thing.

However, astronomical twilight is not long. For example, on the day of the Summer Solstice, astronomical twilight for us along the 40th parallel will only be six hours and 18 minutes long. That is not much time to see, or photograph what you want to.

Well, this doesn't leave much room to talk about Vega, the brightest star in Lyra, the Harp and one of the stars of the Summer Triangle. Vega is the brightest star in Lyra and has the position as the top star of the triangle currently.

It can be located about halfway up the sky from the eastern horizon, and considering the twilight situation, the best time for viewing is after 10 pm local time. Vega--or Alpha Lyrae--is about 25 light-years from Earth.

One curious fact, Vega was at one time (about 12,000 BC) the pole star, or the North Star, as seen from Earth. And due to a process called precession--the wobbling of Earth's axis--it will be the pole star again in the year 13,723, but I am not going to wait around for it.

It is some 40 times brighter than our Sun and about two times the mass.

A couple of interesting things in Lyra, first a star called Epsilon Lyrae located just to the left of Vega. In binoculars you will see it is a double star, in a big enough telescope you will see it is four stars which is why astronomers call it the "Double Double."

Another interesting object is located just below Vega between the two bottom stars of the parallelogram is the fabulous M-57, the Ring Nebula. In binoculars you might see some roundness, but in a larger telescope, with moderate magnification you will see the ring in all its glory.

SKYWATCH: Friday, June 13, full moon. Jupiter is in the lower west, Mars is in the south and Saturn to its left. Bright Venus is the "morning star" just before local sunrise.

NEXT WEEK: Deneb in Cygnus, the Swan, and more astronomical blathering.