Published: 2015-06-30 07:46:00
Updated: 2015-06-30 08:26:24
Posted June 30, 2015
By Tony Rice
This week Jupiter and Venus will be appearing incredibly close in the western sky after sunset. We will even have a little extra time to enjoy this conjunction, the best we will experience until late next summer, but more on that at the end.
The planetary pair will be separated by about 1/3 of a degree, Jupiter above and Venus below to the left. You will be able to cover both with your outstretched pinky (which is about a degree wide). While they appear close and about the same size, this is, of course, a bit of an optical illusion. Jupiter is about 10 times larger and about 10 times further away than Venus.
If you have have access to a small telescope or pair of binoculars this week is a good one to dust them off. Venus will appear as a crescent. Venus and Mercury, the two planets with inferior orbits (between Earth’s orbit and the sun) have phases, like our moon. Jupiter’s moons will also appear through a telescope or binoculars; Ganymede above and to the left, and below (left to right) Europa, Io and Callisto.
If clouds block the view, these planets will continue to appear close for the next week or so. Each evening they’ll separate by another half degree. The next planetary conjunction will occur the last week of August 2016, again with Venus and Jupiter but with their positions swapped thanks to variations in their orbit. The pair will be even closer then, about 1/10th of a degree apart, low of the western horizon.
One second after 23:59:59 Coordinated Universal Time (UTC), atomic clocks world widewill roll to 23:59:60 before 00:00:00. The reason for the disconnect is how we measure time physically (measuring the oscillation of a Cesium-133 atom) and how we represent it astronomical (based on the rotation and orbital periods of the Earth).
While the time it takes for a Cesium-133 atom at ground state to 9,192,631,770 times (1 second) is reliable to a quarter trillionth of a second, the Earth sometimes takes a little less than 86,400 seconds to rotate once. Usually it takes a little more. Recently, that slowdown averaged out to 0.0002 seconds per day.
When those physical and astronomical measurements vary by more than 0.9 seconds, a leap second is added or subtracted. So far, leap seconds have only been added 26 times as of Tuesday’s addition, either on the last day of June or the last day of December.
The International Earth Rotation and Reference Systems Service in Paris, along with the Rapid Service and Predictions of Earth Orientation Parameters at the United States Naval Observatory, issue a directive to add a leap second.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.