Home > Ephemeris Program, Mercury, NASA > 05/07/2015 – Ephemeris – Mercury is at greatest eastern elongation today

05/07/2015 – Ephemeris – Mercury is at greatest eastern elongation today

May 7, 2015

Ephemeris for Thursday, May 7th.  Today the Sun will be up for 14 hours and 30 minutes, setting at 8:54.   The Moon, half way from full to last quarter, will rise at 12:20 tomorrow morning.  Tomorrow the Sun will rise at 6:23.

The tiny planet Mercury has been in the news lately because the MESSENGER spacecraft plunged onto its surface a week ago, after having mapped and studied chemical composition of this planet for four years. Today, for Mercury watchers from the Earth, it reached its greatest angular separation from the sun in its orbit, of 21 degrees just before 1 a.m.   Mercury has always been a tough planet to study, low to the horizon in twilight.  It’s also a tough planet to get to with a spacecraft, being far down the Sun’s gravity well.  MESSENGER took 7 years to get there, bleeding off energy by passing Earth, Venus and Mercury itself to fall into orbit of this little world that was full of surprises.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.


Mercury's orbit

Mercury’s orbit as seen from about 45 degrees north latitude Earth at the greatest eastern elongation at sunset on May 7, 2015. Created using Stellarium.


The other red line is the plane of the Earth’s orbit.  In the spring at sunset it is much closer to vertical than in autumn.  The angle of the ecliptic to the horizon at sunset on the vernal equinox is 90° – (your latitude) + 23.5°.  Here near 45° north latitude it’s 68.5°.  For the sunset at the autumnal equinox the formula is 90° – (your latitude) – 23.5° or 21.5°.  The ecliptic will really lay down making planets close to the direction of the Sun hard to spot.  In the morning sky the ecliptic will be steep at the autumnal equinox and lay down at the vernal equinox.  Thus the best time to spot Mercury, which never strays far from the sun is on late winter and spring evenings and late summer and autumn mornings.  Also note that Mercury’s 7 degree inclination to the ecliptic helps it now.

Also note that we are seeing Mercury’s orbit nearly edge on.  It will be edge-on in a couple of days.  It so happens that a year from now, the morning of May 9th, 2016, for us in the United States, Mercury  will transit, or cross in front of, the Sun.  Three years ago this June we witnessed the extremely rare transit of Venus.  The transit of Mercury isn’t as spectacular or rare, but it’s rare enough.


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