Planets and the Moon: what is different
1The lesson in plain words
Planets and the Moon are reduced like the Sun, but their almanac data need extra corrections. The Moon's large parallax makes its altitude correction big.
The four planets
The almanac tabulates four navigational planets: Venus, Mars, Jupiter and Saturn. They are often the brightest objects in twilight, and Venus can be seen in daylight. Like the Sun, each has its own GHA and Dec every hour on the daily pages. They do not keep fixed SHAs like stars.
v and d
The increments tables assume a standard rate of change of GHA. A planet's actual rate differs slightly; the difference per hour is v, printed at the foot of its column. Correct GHA with v for the minutes past the hour, and Dec with d, as for the Sun. Venus's v can be negative. Altitude corrections are as for stars, with small additional ones for Venus and Mars.
The Moon
Parallax is the change in the Moon's apparent direction between Earth's centre and your position. Horizontal parallax (HP) is this angle when the Moon is on your horizon: about 54′–61′, listed hourly. Its v and d are also large. Use the Moon tables with apparent altitude, HP and the observed lower or upper limb—the bottom or top edge. They combine refraction, semi-diameter and parallax.
A daytime fix
When the Moon and the Sun are both up, with a good angle between their azimuths, you can cross a Sun line with a Moon line: a fix by day without waiting for noon. Around full Moon, the Moon is often too close to the horizon or opposite the Sun.
This evening
On 24 November 2026, the Moon was full at 1453 UT, with HP about 60.8′. By evening twilight it had just risen in the east-north-east. Saturn stood about 47° up in the east-south-east: a good fourth line. A bright Moon can light the horizon under it after twilight, but treat such sights with caution.
2Key data
- Navigational planets
- Venus, Mars, Jupiter, Saturn
- Planet v
- Hourly excess of GHA over the standard rate; Venus can be negative
- Moon HP
- About 54′ to 61′; listed hourly on the daily pages
- Moon tables
- Enter with apparent altitude and HP; lower or upper limb
- 24 Nov 2026
- Full Moon 1453 UT, HP about 60.8′; Saturn about 47° up
3Key points
- 1
Four planets: Venus, Mars, Jupiter, Saturn, each with hourly GHA and Dec.
- 2
Correct planet GHA with v and Dec with d; Venus and Mars get small extra corrections.
- 3
The Moon's HP is large: use its own tables with apparent altitude, HP and limb.
- 4
A Moon line can cross a Sun line for a fix by day.
4Frequent mistakes
Wrong: Find a planet's GHA from GHA Aries and a fixed SHA, like a star.
Right: Planets move against the stars. Take each planet's own GHA and Dec from its column, with v and d.
Wrong: Correct a Moon sight with the star tables plus semi-diameter.
Right: The Moon's parallax is large and changes with HP. Use the almanac's Moon tables with apparent altitude, HP and limb.
Wrong: Ignore v because it is printed only at the foot of the column.
Right: Apply v for the minutes past the hour. For the Moon it can exceed 10′ an hour, worth several miles.
5Practice case & exam keys
Mid-morning, the Sun bears 120°T and a half Moon bears 230°T, about 40° up. You have one Sun line and no noon sight for three hours. What can you do?
Show the answer
Take a Moon sight now. The azimuths differ by about 110°, a good angle. Reduce it with the Moon's own GHA, v, Dec and d, and correct the altitude with the Moon tables using HP and the limb you observed. Advance the earlier Sun line to the Moon sight's time, then cross the two lines for your daytime fix.
Exam keys
- Navigational planets: Venus, Mars, Jupiter, Saturn; own GHA and Dec hourly.
- v: hourly excess of GHA over the standard rate (Venus can be negative); d for Dec.
- Moon: HP about 54′–61′; Moon tables with apparent altitude, HP, lower/upper limb.
- Sun–Moon fix by day when the azimuths differ well.
Finish this unit
When you've read it, answer three questions. Get all three right to finish the unit; opening the page alone doesn't count.
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