The 7 fields and concepts you'll meet in the response — defined in plain English, each with a real example value.
The angle of the moon above the horizon, measured in degrees from 0° (horizon) to 90° (directly overhead).
When the moon is below the horizon, altitude is negative. Maximum lunar altitude depends on the observer's latitude and the moon's position in its orbit. In mid-latitudes, the moon can reach 60-70° altitude.
Examplealtitude: 35.8 (moon is 35.8° above horizon)
The compass direction of the moon measured in degrees from true north (0° = north, 90° = east, 180° = south, 270° = west).
Unlike the sun, the moon's rising and setting azimuths vary dramatically throughout its monthly cycle due to its orbital inclination. The moon can rise anywhere from northeast to southeast depending on its orbital position.
Exampleazimuth: 145.2 (moon is southeast)
The angle between the great circle through a celestial object and the zenith, and the hour circle of the object.
For photographers and telescope users, parallactic angle describes how the moon appears rotated relative to "up" in the image. At the horizon, the moon may appear tilted; at the meridian, it appears level. This is crucial for image composition and for operating field derotators.
ExampleparallacticAngle: -12.5 (moon tilted 12.5° counterclockwise)
The point in the moon's orbit when it is closest to Earth, approximately 356,500 km away.
At perigee, the moon appears about 14% larger and significantly brighter than at apogee. Perigee occurs once per lunar orbit (about every 27.5 days). When a full moon coincides with perigee, it's called a supermoon.
Exampledistance: 357000 (near perigee)
The point in the moon's orbit when it is farthest from Earth, approximately 406,700 km away.
At apogee, the moon appears smaller and dimmer than average. A full moon at apogee is sometimes called a "micromoon." The difference between apogee and perigee full moons is noticeable in photographs but subtle to the naked eye.
Exampledistance: 405000 (near apogee)
A full moon that occurs when the moon is at or near perigee (closest approach to Earth).
The term "supermoon" was coined in 1979. Technically, a supermoon is a full moon within 90% of its closest approach. Supermoons appear about 14% larger and 30% brighter than average full moons. There are typically 3-4 supermoons per year.
ExampleA full moon at distance 357,000 km would be a supermoon
The percentage of the moon's visible surface that is lit by the sun (0-100%).
Illumination is 0% at new moon and 100% at full moon. The term is often used interchangeably with "phase" but is more precise. A "half moon" (first or last quarter) has exactly 50% illumination. Illumination affects exposure settings for lunar photography.
Exampleillumination: 67.3 (67.3% of visible surface lit)
