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The Sky This week, 2015 June 30 - July 7

A leap second, the latest sunset, aphelion, and Saturn!
Saturn, 2015 June 30, 03:17 UT
Imaged with the U.S. Naval Observatory's historic 30.5-cm (12-inch) f/15
Clark/Saegmüller refractor

The Moon wanes from the full phase in the late evening sky this week, coursing her way through the dim stars of the autumnal constellations. Last Quarter occurs on July 8th at 4:24 pm Eastern Daylight Time. Luna’s bright glow will initially drown out the faint light of the summer Milky Way, but by then end of the week you should be able to see the brightest portions of the galaxy from dark locations.

The final day of June this year will be different from most with the addition of a positive "leap second" that will occur during the last minute of the day as measured by Coordinated Universal Time (UTC, which will be just before 8:00 pm EDT). For most of us one second isn’t all that much to worry about, but folks, especially network administrators, this special second can cause no end of problems. Leap seconds exist due to the inconsistent rotation of the Earth, which is gradually slowing down. Time is now kept by atomic clocks, which are incredibly stable devices that won’t gain or lose more than a second in something like 3 million years. The day, as now measured with these precise clocks, is exactly 86,400 seconds long, but the time that it takes the Earth to rotate once on its axis is about 86,400.002 such seconds. The small difference between these two "days" adds up over time, so after roughly 500 days the two time-scales differ by about one second; thus we essentially stop our atomic clocks for one second to let the Earth "catch up". This dual time-scale, when first implemented in 1972, satisfied the precise requirements of physicists who desired a non-varying second, while astronomers could use a time-scale based on the Earth’s rotation for their Earth-based measurements of celestial objects. The two time-scales peacefully coexisted until the dawn of the 21st Century when increasingly more complex computer networks began suffering failures for not properly accounting for the leap second. We’ll see what happens this time around, but don’t be surprised if you have a few glitches in your Internet connections as the second is added.

We’re now a week past the summer solstice, but we are experiencing the year’s latest sunsets as June rolls into July. Here in Washington Old Sol slips below the horizon at 8:38 pm EDT at the beginning of the week. He’ll set two minutes later by July 7th.

We’re also at the time of year when the Earth reaches aphelion, its most distant point in its orbit from the Sun. The exact moment will be at 3:40 pm EDT on July 6th. At that moment we’ll be just over 152 million kilometers (94, 506,507 miles) from the day-star.

Venus and Jupiter are in the middle of a spectacular dance in the early evening sky. They are closest together on June 30th, but remain in close proximity for the rest of the week. Venus will appear to pass Old Jove and pull a few degrees ahead of him over the rest of the week. Both objects are now losing ground to the advancing Sun. By the end of the week they will set at the end of evening twilight.

Saturn transits the meridian just after 10:00 pm this week. This is the time when the ringed planet reaches his highest elevation above the southern horizon and is the best time to get a look at him in the telescope. You can see him quite easily in fading twilight by 9:00 pm, and you’ll have several hours to enjoy the view before he begins to sink into the summer horizon haze.


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