Full harvest Moon (Chris Vaughan, Starry Night Education)

This Week’s Sky: September 28 to October 4, 2020

Mercury reaches greatest eastern elongation, we see the full harvest Moon and as it starts waning the Moon meets Mars.

Thursday, October 1 evening – Mercury at greatest eastern elongation

Mercury at greatest eastern elongation (Chris Vaughan, Starry Night Education)

In the western sky on the evening of Thursday, October 4, Mercury (orbit shown in red) will reach its widest separation, 26 degrees east of the Sun. With Mercury positioned well below the evening ecliptic (green line), this appearance of the planet will be a poor one for Northern Hemisphere observers, but offer excellent views for observers near the equator and in the Southern Hemisphere. The optimal viewing times at mid-northern latitudes fall around 7:15 p.m. local time. Viewed in a telescope (inset) the planet will exhibit a waning, half-illuminated phase.

Thursday, October 1 at 21:05 GMT – Full harvest Moon

Full harvest Moon (Chris Vaughan, Starry Night Education)

Because this full Moon occurs closest to the autumnal equinox in 2020, it is also the harvest Moon. On the evenings around its full phase, the Moon usually rises about 50 minutes later each night. But the shallow angle between the horizon and the evening ecliptic on dates around the equinox causes the Moon to rise at almost the same time each night — only delayed by 10-20 minutes, depending on your latitude. This lunar phenomenon traditionally allowed farmers to work longer into the evening under bright moonlight when the crops were ready to harvest — hence the name.

Friday, October 2 pre-dawn – Venus kisses Regulus

Venus kisses Regulus (Chris Vaughan, Starry Night Education)

When very bright Venus rises in the east at about 3:45 a.m. local time on Friday, October 2, it will be positioned less than a finger’s width above (or 41 arc-minutes to the celestial west of) the bright, white star Regulus in Leo. The planet and star will appear together in the field of view of binoculars, or in a backyard telescope at high magnification (red circle). At closest approach, at 23:00 GMT, observers in western Asia can see Venus only 5 arc-minutes from the star. On the following morning, Venus’ orbital motion eastward will lower it to a half finger’s width below Regulus.

Friday, October 2 all night – Bright Moon meets Mars

Bright Moon meets Mars (Chris Vaughan, Starry Night Education)

When bright, reddish Mars rises in the eastern sky at around 7:45 p.m. local time on Friday, October 2, the gibbous, waning Moon will be positioned just two finger widths to the lower right (or 2.25 degrees to the celestial south) of the red planet. The Moon and Mars will be close enough to one another to see them together in binoculars (red circle) all night long. By midnight, the diurnal rotation of the sky and the Moon’s eastward orbital motion will carry the Moon to just a finger’s width below Mars. At sunrise on Saturday morning, Mars will sit three finger widths to the Moon’s lower right in the western sky — allowing you to find the magnitude -2.52 planet in the brightening western sky with binoculars. Early on Saturday morning, the Moon will occult Mars for observers in southern and southeastern South America, most of western Antarctica, the Ascension Islands, and southwestern Africa.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.