Mercury orbits inside Earth’s orbit, closer than Venus, so it is an inferior planet. From here the disk runs the same range Venus does — crescent, half, gibbous, and the far-side near-full — because we look at the planet from outside its orbit. Inferior conjunction puts Mercury between Earth and the Sun. Superior conjunction puts it on the far side, near full, and still near the Sun in the sky. The greatest elongations are as far from the Sun as these circles allow, and that angle is tighter than Venus’s because the orbit is smaller. NASA’s Basics of Space Flight is the vocabulary: elongation is the apparent angle from the Sun, and an inferior planet can show nearly the whole phase range. NASA’s skywatching notes add the practical consequence: Mercury is visible for only a couple of weeks, or even a few days, at a time, just after sunset or just before sunrise. Pair it with Venus, the wider inner orbit, and with the phase hub.
Educational schematic — circular orbits, one plane, disks exaggerated so a phone can see them. Earth is held on the right. Both worlds still orbit the same way; Mercury is the faster one, so the marker walks counterclockwise. Circle radii use the fact-sheet ratio 0.387. That sheet lists eccentricity 0.2056 and inclination 7.004°, so a real greatest elongation is not this single tangent, and a real inferior conjunction usually misses the Sun’s face. This coplanar cartoon goes dark at the lineup. Not an ephemeris. Not a date. Not a reason to look at the Sun. This page does not invent a magnitude or an elongation range.
Interactive · From Earth
North up · Earth held on the right · circles at the fact-sheet ratio
As seen from Earth · east to the right · ghost ring is the largest size on these circles
Sun
Mercury
Earth · viewpoint held
Line of sight
0 d
Literacy · words for the phase
Inferior planet
NASA’s Basics of Space Flight: Mercury and Venus are inferior planets because their orbits are closer to the Sun than Earth’s. They stay near the Sun in the morning or evening sky. Mars, on the Mars phase desk, is superior and does not run this crescent-to-full range.
Phase
The sunlit fraction we see. The same chapter: an inferior planet can show nearly the whole range, from the dark “new” phase, through a crescent, to gibbous, approximating full on the far side of the Sun. The Moon’s phases are a body going around us. These are a body going around the Sun inside our orbit.
Inferior conjunction
Mercury approximately between Earth and the Sun. Basics of Space Flight: if it crosses the Sun’s face, that is a transit. The fact sheet’s 7.004° inclination is why the usual case is a thin crescent in the glare. This slider’s zero is a schematic lineup, not a transit date.
Superior conjunction
Earth and Mercury on opposite sides of the Sun. The phase is near full, the disk on these circles is as small as it gets, and Mercury is still close to the Sun in the sky.
Elongation
The apparent angle from the Sun, in NASA’s space-flight chapter. Eastern elongation is the evening side on this drawing; western elongation is the morning side. The presets sit on the tangent of these circles. They are not an almanac.
Synodic period
The repeat of the same Sun–Earth–Mercury alignment. The fact sheet lists 115.88 days. Sidereal orbit periods on that sheet are 87.969 days and Earth’s 365.256 days. The slider walks one schematic lap. It does not name the next evening apparition.
Sources · public NASA pages
Cited, not invented
NASA NSSDCA — Mercury Fact Sheet — semimajor-axis ratio 0.387; sidereal orbit periods 87.969 days (Mercury) and 365.256 days (Earth); synodic period 115.88 days; mean orbital velocity 47.36 km/s (Mercury) and 29.78 km/s (Earth); eccentricity 0.2056; inclination 7.004°. Those means set the circles. The sheet’s V(1,0) magnitude is not an apparent magnitude for this slider, so it is not used.
NASA Science — Basics of Space Flight, Chapter 1 — inferior planets orbit closer to the Sun than Earth and stay near the Sun in the morning or evening sky; elongation is their apparent angle from the Sun; they show phases from dark “new,” through crescent, to gibbous, approximating full on the far side; inferior conjunction is the planet approximately between Earth and Sun; superior conjunction puts Earth and the planet on opposite sides of the Sun.
NASA Science — Planetary alignments and planet parades — Mercury, completing its orbit in just 88 days on that page’s wording, is visible for only a couple of weeks, or even a few days, at a time, just after sunset or just before sunrise. This slider counts the fact sheet’s 87.969-day sidereal period and 115.88-day synodic period, not a new conversion of “88 days.”
NASA Science — Hello, Mercury! (MESSENGER, 2009) — four days before the third flyby, Mercury appears as a sunlit crescent as the spacecraft approaches. That crescent is a spacecraft viewpoint. It is not an Earth-based elongation, and the miles in the caption are that exposure, not a distance this slider prints.
Classroom picture of an inner orbit and the phase it shows from Earth. Not an ephemeris and not a pointing aid. Days on the slider are a coordinate of these circles, counted from a schematic inferior conjunction. The phase angle is the Sun–Mercury–Earth angle on the cartoon. Periods, the 0.387 ratio, the eccentricity, and the inclination are the ones NASA publishes on the pages above. Earthrise preview: NASA / Bill Anders (public domain).