This is Dudley Observatory’s Skywatch Line for Friday, July 22, through Sunday, July 24, written by Sam Salem.
The waning Gibbous Moon is 91% illuminated on Friday. The Moon rises at 10:05pm on Friday; 10:40pm on Saturday; 11:13pm on Sunday night. The Moon reaches a descending node at 7:09am on Saturday. The lunar nodes are the points where the Moon’s path in the sky crosses the ecliptic, the Sun’s path in the sky. The descending node is where the Moon crosses from north of the ecliptic to south of the ecliptic. The ascending node is where it crosses from south of the ecliptic to north of the ecliptic. The lunar orbit is inclined by about 5.1 degrees to the ecliptic. Hence, the Moon can be about 5 degrees north or south of the ecliptic. Eclipses occur only near the lunar nodes. Solar eclipses occur when the passage of the Moon through a node coincides with the new Moon. Lunar eclipses occur when passage coincides with the full Moon.
One hour after sunset, Jupiter, Mars, and Saturn are past the meridian and starting to set. Farthest west is Jupiter. It sets at around 11pm, just as astronomical twilight ends. As twilight fades, Mars, of magnitude -1.0, shines in the south-southwest. Mars’ disk shrank to 14 arc-seconds, which is almost ¾ the size it was during its close approach at the end of May. Farthest east is Saturn, of magnitude 0.3, northeast of Antares, the brightest star in Scorpius. Saturn is the highest of the three planets in the sky. Saturn will be a good planet to observe this weekend even in the Moon lit sky. Saturn’s brightest moon, Titan, shines at magnitude 8.4 and is the brightest point of light near the planet. Rhea, Tethys, Dione, and Encleadus are four additional Saturnian moons you could see if you have an 8-inch or larger telescope.
On Friday night-Saturday morning, the waning gibbous Moon will occult Neptune and star Lambda Aquarii for about an hour. Because of the Moon brightness at that time, seeing the disappearance of the star will be relatively easier than seeing Neptune, magnitude 7.8, disappear. However, both of them will be visible during their reappearance at the Moon’s dark limb. An 8-inch or large scope would minimize the scattered light and maximize the chance to glance at the planet. Neptune returns to view along the Moon’s southeastern limb around 12:35am on Saturday.
The German astronomer and mathematician, Friedrich Wilhelm Bessel, was born in July 22, 1784. Bessel was the director of Frederick William III of Prussia’s new Königsberg Observatory and professor of astronomy. His monumental task was determining the positions and proper motions for about 50,000 stars, which allowed the first accurate determination of interstellar distances. Bessel was the first astronomer to measure the distance of a star, by parallax in 1838.