Tuesday morning, May 23rd the thin 26.7-day old waning crescent Moon will be surrounded by planets, most of which will be difficult to see given the time of day and how faint each one is. Except for Venus.
On Wednesday May 17th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 25.8o. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows. Even though it sounds confusing at western elongation for either Mercury or Venus the inner planet will be to the right of the Sun as we view them, meaning that at western elongation an inner planet rises in the east before the Sun rises. And at eastern elongation with the inner planet on the left side of the Sun the inner planet follows the Sun across the sky setting after the Sun sets.
From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – out to the left (east) from the Sun to eastern elongation, then reverse and move westward (inferior conjunction) between the Earth and the Sun to western elongation. From there the inner planet moves eastward going behind the Sun (superior conjunction) and eventually reappearing on the eastern side of the Sun for an eastern elongation. Repeat over and over – do not stop!
Mercury is visible in the morning skies before sunrise as this graphic shows.
Thursday April 20th the innermost planet Mercury reaches inferior conjunction. At inferior conjunction Mercury will move between the Earth and the Sun – much like the position of the Moon at new phase. The graphic to the right shows the planet positions relative to the Earth and Sun for both inner planets and outer planets.
While at this inferior conjunction Mercury will not be directly in line with the Earth and the Sun – on the ecliptic. Mercury has an orbital inclination of 7o with respect to the ecliptic. So like our Moon, Mercury during each complete orbit, will cross the plane of the ecliptic moving north (ascending node) and also moving south (descending node). In this situation Mercury is north of the ecliptic and 6 days away from its descending node on April 26th.
Sunday April 9th the innermost planet Mercury reaches a point in its orbit around the Sun where it moves toward the west as we view Mercury from the Earth. Typically when one hears this the term Retrograde Motion comes to mind, but with regard to an outer planet (relative to Earth). However while an outer planet’s retrograde motion is apparent the westward motion of an inner planet, Mercury or Venus, is very real. For Mercury or Venus retrograde motion begins around the time of eastern elongation and continues through inferior conjunction to around the time of western elongation. For Mercury this particular retrograde motion will end next month on the 17th.
Tuesday evening and Wednesday evening March 28th and 29th there will be opportunities to observe a young and thin waxing crescent Moon. Tuesday as the thin waxing crescent Moon sets it will be approximately 0.94 day young. The following evening, Wednesday, the setting Moon will be approximately 1.94 days young.
Tuesday March 7th the innermost planet Mercury reaches superior conjunction – on the opposite side of the Sun from the Earth. For those that are curious, Mercury at superior conjunction is approximately 1.363 AU (126,698,765 miles; 203,901,898 km) from the Earth – the combined distance of the Earth to Sun distance plus the radius of Mercury’s orbit.
Mercury is not visible while in conjunction with the Sun but within the next week or so Mercury will reappear on the east side of the Sun and start becoming visible over the western horizon at sunset.
(posting this ahead of time – on the road back to Quito and no Internet for a couple of days)
Sunday morning January 29th the innermost planet Mercury will be about 1o from the the former outermost planet and now a Dwarf Planet, Pluto. Mercury with an apparent magnitude of -0.18 far outshines the 14.19 apparent magnitude of Pluto.
These two graphics show the sky as viewed from Quito Ecuador at 0o latitude, and my home latitude of approximately 40o North.