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Active skies in May and June
By Rick Nowell
Some interesting events have occurred and are occurring this May and June for early morning and late night star viewing.
The Aquariid Meteor Shower (Thurs, May 5): After three months of quiet skies, meteor activity increased towards April-May. The full Moon on April 22 ruined the Lyrid meteor shower (a medium shower of 18 meteors per hour). But both the eta-Aquariid shower maximum on Thursday, May 5 with an average of 40 meteors per hour at peak, and the smaller eta -Lyrids with a peak on Tuesday, May 10 (about three meteors per hour) will occur on dark, moonless nights
The Transit of Mercury (Monday morning, May 9): the tiny dot of Mercury moved across the face of the Sun on the morning of Monday, May 9 at 8:58 a.m., lasting until 12:40 p.m. It’s so small (12 arcseconds* wide against the Sun’s 0.53 degrees width) you’d need a telescope to see it, and it’s risky, you’d have to take precautions not to be blinded, by using proper solar screens or project the image on a screen. The photo below was taken by NASA’s Trace satellite on May 7, 2003, each dot is Mercury at 7.5 minute intervals, moving across the top part of the Sun’s disc. The wobble in Mercury’s line is from the satellite moving in a polar orbit around the Earth, moving “up/down” every 97.5 minutes. It took Mercury about 5.25 hours to cross the face of the Sun here.
Estimating the Distance to Mercury: From the angle of this wobble shift up/down it is possible to estimate the distance from Earth to Mercury using triangulation. NASA mentioned the satellite orbital radius was 6,698 km (the radius of the Earth 6,378 km plus the satellite’s compensated height above the Earth of 320km on that date).  Looking close and measuring, Mercury appears to move up and down against the Sun by 0.0094 degrees (just 33.84 arcseconds).  If we put these sizes and angles in a triangle, we can solve for the distance to Mercury, x.   The diagram below shows more details, but it’s not to scale.  The answer here is 82 million kilometres, which is close to the 77 million it actually is. Measuring the angle from this photo is difficult for small and distant Mercury, you get more accurate results when Venus transits since it’s closer and would move up/down more.  That was done in 1761 from widely separated locations on the Earth.  Nowadays they would bounce a radar signal off of the planet to get more precise results.
[*Note arcseconds: Like a clock, a degree can be divided into 60 minutes of arc, and each minute of arc can be divided into 60 seconds of arc. So 12 arcseconds is 12/60 of a minute of arc; or (12/3600) of a degree. The full Moon and the Sun are about half a degree wide, or 30 minutes of arc wide. Planets appear very small in comparison. ]
Right is a recent photo (taken April 19) from our 14” diameter Meade Schmidt-Cassegrain reflecting telescope with an USB video cam.  Air turbulence made them a bit blurry. The photos approximately represent what you’d see through the college’s telescopes, although the view through an eyepiece tends to be sharper–the eye works better than the camera sometimes.
Mars will be its brightest orange colour and closest to the Earth on Sunday, May 22 around 1 a.m.  It will be fairly close, at 0.51AU away. Look in Scorpio, near the scorpion’s reddish heart Antares. In a telescope Mars appears as a disc 18 arcseconds wide. Next time around on July 27, 2018, it will be even closer and bigger, appearing 24 arcseconds in diameter.  In the photo, some blurry markings on Mars are visible, the South Pole a white patch below. This will get much clearer in a month when Mars gets closer.
The distance between the Earth and Mars at opposition (when the Earth is directly between the Sun and Mars) can be as close as 0.373 Astronomical Units (34.6 million miles or 55.7 million kms), and can be as far as 0.678 AU (63 million miles or 101 million kms). The Red Planet’s apparent magnitude at opposition varies from -2.9 (closer ones) to -1.2 (further ones), thus the planet can be five times brighter at the closer oppositions than the further oppositions.
Jupiter (lead image) already reached opposition on March 8, at a distance of 4.44 AU away from the Earth. It’s still pretty close: in a telescope it will appear 44 arc seconds wide.
Here you can see a hint of Jupiter’s red spot, to the left just below the equator. And the pale orange stripes against dull greys. This spot moves quickly to the right, and crosses in just five hours. Jupiter’s four bright moons aren’t shown here, but your eye would see four sparks in a line as well.  The innermost moon Io takes just 42 hours to go around, so if you look at it again an hour later, you’ll notice movement.
Saturn, a yellowish white bright planet (right), is to the left of Mars, low to the South at 4am, making a triangle with orangey Antares below.  Opposition occurs on June 3 2016 at 9.0 AU away from the Earth.  Although it is twice as far as Jupiter, its rings are so large it appears nearly as wide, at 42 arcseconds.
Looking at the photo, notice Saturn’s rings are currently tilted down by 26 degrees. They tilt up and down, repeating every 15 years, their maximum tilt was in March 2003, and in March 2009 they went invisible since they were straight on. In the photo you can just make out the Cassini division, a black line between the outer rings and inner.
Starmap showing Saturn (SAT) and Mars (MAR) on April 18 at 5 a.m. Note they are on the Milky Way on the Southern horizon, above Scorpius and the bright reddish star Antares. Â See also the little teapot asterism of Sagittarius to its left–behind it lies the core of our Milky Way galaxy.
This sums it up. A map of the planets in their orbits, with their positions marked. The planets all move counterclockwise, and the inner ones move faster than the outer ones. Note (purple) Mercury is quickly moving between the Sun and the (green) Earth. And the (green) Earth is moving between the Sun and (red) Mars towards “opposition.”  [The diagram is generated from The Sky Astronomy software.]
The College of the Rockies is doing their Astronomy 100 course again in September. We have a bunch of Nikon cameras and medium telescopes that allow students to take photos of stars and planets.
– Rick Nowell is the Astronomy Lab Tech at College of the Rockies.