All solar orbiting objects will at some point in their respective orbit cross the plane of the ecliptic. The ecliptic is actually the Earth’s orbit around the Sun, however from our perspective on the Earth it appears as if the Sun is moving eastward along the ecliptic.
On April 12th the inner planet Venus will be at its ascending node as it crosses the ecliptic moving north. And the next day, April 13th, Mercury will be at its descending node moving south as it crosses the ecliptic.
Sunday April 1nd 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). For this inferior conjunction Mercury will be north of the ecliptic.
Over the next week or so the two inner planets will pass by each other coming the closest on the 19th when the two will be less then 4o from each other. With only casual observation one should notice that both planets are moving with respect to each other, but in opposite directions. Mercury is recently past its eastern elongation and is now moving westward, in retrograde, toward the Sun and inferior conjunction. On the other hand, or orbit, Venus is moving eastward out from a recent superior conjunction, opposite side of the Sun, toward its own respective eastern elongation.
This animated graphic shows Venus and Mercury over a period of several days from March 16th to the 29th of March. The time is set for 7:15 pm CDT and in the first several frames the planets are first shown as they would appear at 7:15 pm, then I added labels, then their respective orbits. To make the animation easier to see I also turned off daylight, and then finally the labels were turned off then back on at the end.
Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to return to bobs-spaces.
Sunday evening March 19th the thin 1.5-day young waxing crescent Moon, and the two inner planets, Venus and Mercury, will be over the western horizon about 1 hour after sunset local time. All three will fit within the width of a 7×50 binoculars field of view (about 7o).
On Thursday March 15th Mercury, the innermost planet, will reach its orbital position known as greatest eastern elongation. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows.
From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – from superior conjunction, behind the Sun, out to the left (east) from the Sun to eastern elongation, then reverse and move westward through (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!
Currently Mercury is visible over the south to western horizon at sunset shining with an apparent magnitude of -0.14. About 3o from Mercury is the planet Venus with a much noticeable apparent magnitude of -3.91.
On the evenings of March 3rd, 4th, and 5th the two inner planets Mercury and Venus will become visible, but low, over the western horizon shortly after sunset local time. Due to its faster orbital speed Mercury will pass Venus as this animated graphic is showing. At their closest the two will be separated by about 1.5o.
This should make for good viewing with binoculars and for a great photo opportunity. The two are close enough to just barely fit in a 25 mm eyepiece field of view on a 6″ reflector..
Saturday February 17th 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 1.384 AU (128,650,837 miles; 207,043,453 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.