This Week’s Sky at a Glance, 2021 June 26 – July 3 ~by Curt Nason
Wednesday is International Asteroid Day, an annual event sanctioned by the United Nations in 2016 to raise awareness of the potential hazards and benefits of asteroids. The first asteroid, Ceres, was discovered on January 1, 1801 by Giuseppe Piazzi at the Palermo Astronomical Observatory. A few decades previous an astronomer had developed a mathematical relation that seemed to fit the relative distances from the Sun of the six known planets (out to Saturn). According to this relation there was a planet missing between Mars and Jupiter, and Ceres was in the right area. Over the next six years three more were discovered (Pallas, Juno and Vesta) at roughly the same distance, and astronomers were questioning whether these should still be considered planets.
On June 30, 1908, a 60-metre wide stony asteroid (or a somewhat larger comet) exploded at an altitude of eight kilometres over the sparsely populated region of the Tunguska River in Siberia, about 700 km northwest of the northern tip of Lake Baikal. At 7:17 am local time a tongue of flame split the sky, followed by loud bangs, ground-shaking tremors and a hot wind of hurricane force. A seismic event was recorded 900 km south, and a microbarograph in England recorded a pressure event five hours later and again a day after that. Expeditions were led two decades later by Leonid (great name for a meteorite hunter) Kulik to locate and interview eye witnesses and to locate the crater and meteorites. No crater or meteorites were found, but there was an area of 2100 square kilometres where trees were blown down in a radial pattern. Those trees in the midst of the destruction remained standing with their limbs stripped.
Ceres, by far the largest asteroid, was reclassified as a dwarf planet in 2006 at the same time as Pluto. To celebrate Asteroid Day, try to locate an asteroid in the night sky. It will look like a faint star in a telescope, and a good star map will be needed to distinguish one from the background stars. The traditional method is to carefully sketch the star field and return the next clear evening to see which one has changed position relative to the others. The Heavens-Above website has wide-field and detailed inset maps for the brightest asteroids, and Vesta is currently the only one bright enough to be seen in most binoculars. The inset map is about the size of the field of view seen with common binoculars.
This Week in the Solar System
Saturday’s sunrise in Moncton is at 5:29 am and sunset will occur at 9:14 pm, giving 15 hours, 45 minutes of daylight (5:38 am and 9:16 pm in Saint John). Next Saturday the Sun will rise at 5:33 am and set at 9:13 pm, giving 15 hours, 40 minutes of daylight (5:41 am and 9:15 pm in Saint John).
The Moon is below Saturn in morning twilight Sunday, it rises below Jupiter around midnight Monday evening, and is at the third quarter phase on Thursday. Mercury rises an hour before sunrise midweek but it will be brighter during the second week of July. Venus sets around 10:50 pm midweek, approximately ten minutes before Saturn rises and 20 minutes before Mars sets.
Questions? Contact Curt Nason.