This is Dudley Observatory’s Skywatch Line for Friday, July 13 through Sunday, July 15, written by Sam Salem.
On Friday, the Sun rises at 5:29am and sets at 8:33pm. The new Moon occurred on Thursday at 10:48pm. As twilight fades on Saturday, try to locate the tiny crescent Moon over Mercury very low in the west and to the lower right of Venus. Your best view may be about 45 minutes after sunset. The waxing crescent Moon will be about 2 degrees right of Venus on Sunday early evening. This is close enough for the pair to look good in binoculars or a small scope used at low power.
All five naked-eye planets are viewable before midnight this weekend. Mercury is at greatest elongation from the Sun on Thursday. Mercury sits roughly 6 degrees below and right of Venus, low in the west-northwest during twilight. Due to the shallow angle the ecliptic makes with the horizon at this time of year, Mercury is now slightly lower at dusk than it was at the end of June. Jupiter is already past the meridian as twilight fades in the southwest. At the same time, Saturn is ascending in the south-southeast. Saturn culminates soon after midnight. Mars rises in the southeast horizon around 10:30pm. Mars has brightened considerably as it nears its July 31 close approach. This week Mars shines at magnitude –2.5 and features a disk spanning 22.8 arc seconds. A global dust storm has rendered most Martian surface details indistinct or invisible. Hopefully, the storm will abate soon and Mars observers will be rewarded with best views of the planet since 2003.
Watch satellites slowly drifting across the sky during deep twilight this weekend. You will be able to see them as the Sun is still shining brightly there at altitude of these satellites. The most known human-made satellite object currently in orbit is the International Space Station (ISS). It circles the Earth every 90 minutes at an altitude of 400 kilometers. If you’ve ever wondered about a “bright light” moving west to east at night, there’s a good chance it’s the ISS. The ISS isn’t the only brightly reflective space satellite. Iridium satellites can also put on an impressive display. Unlike the space station, Iridium satellites produce a relatively brief flare that fades away after a few seconds. This is a particularly good weekend for sighting the space station in the morning sky. If you want to catch the ISS or an Iridium flare, there are a number of on-line resources. Use NASA’s Spot the Station site. Select your location and the site produces a timetable of upcoming ISS passes. The Heavens Above site lists ISS passes as well as the timings for Iridium flares. The site provides ISS Interactive, a real-time 3D Visualization ISS-eye view of the Earth.
One hour after sunset this weekend, as twilight fades further and the stars are coming out, try to locate the two brightest stars of summer, Vega and Arcturus. They are about equally near the zenith, The very pale bluish white Vega sits toward the east. The pale yellow-orange Arcturus shines toward the southwest.
Sunday marks the 53rd anniversary of the Mariner 4 closest approach to Mars. The satellite sent a transmission of the close-up photograph of Mars, consisting of 8.3 dots per second of varying degrees of darkness. The transmission lasted for 8.5 hours. The satellite was 134 million miles away from earth and 10,500 miles from Mars. The spacecraft provided key information about how to safely deliver future missions to the Martian surface. Far outlasting its planned eight-month mission, the spacecraft lasted about three years in solar orbit, continuing long-term studies of the solar wind and making coordinated measurements with Mariner 5 spacecraft.