12° above the horizon at

sunset on

9 Dec 2023.

Nov–Dec 2023 evening apparition of Mercury

The table below lists the altitude of Mercury at

sunset
over the course of the apparition. All times are given in Los Angeles local time.

Mercury will fade rapidly towards the end of the apparition as it heads towards inferior conjunction,
when it will pass between the Earth and Sun. At inferior conjunction, the planet turns its unilluminated side
towards the Earth, and so appears as a thin, barely illuminated crescent.

Since Mercury can only ever be observed in twilight, it is particularly difficult to find when it is in a thin
crescent phase. Thus, it will be significantly easier to see in the days before it reaches its highest point
in the sky than in the days after.

Altitude of Mercury at sunset

A graph of the angular separation of Mercury from the Sun
around the time of greatest elongation is
available here.

Apparitions of Mercury

Observing Mercury

Mercury’s orbit lies closer to the Sun than the Earth’s,
meaning it always appears close to the Sun and is lost in the Sun’s
glare much of the time.

It is observable for

only a few weeks
each time it reaches greatest separation from the Sun – moments referred
to as greatest elongation.

These apparitions repeat roughly once every

3–4 months,

taking place alternately in the morning and evening skies, depending whether

Mercury
lies to the east of the Sun or to the west.

When it lies to the east, it rises and sets a short time after the Sun and is
visible in early evening twilight. When it lies to the west of the Sun, it
rises and sets a short time before the Sun and is visible shortly before
sunrise.

However, some times of the year are more favourable for viewing Mercury than others. From Los Angeles, it reaches
a peak altitude of between

11°

and

22°

above the horizon at sunset
during each evening apparition,
depending on the time of year. During its Nov–Dec 2023 apparition,
it will peak at

12° above the horizon at

sunset on

9 Dec 2023.

This variability over the course of the year is due to a combination of two factors.

The inclination of the ecliptic to the horizon

At all times, Mercury lies close to a line across the sky
called the ecliptic, which is shown in yellow in the planetarium above. This line
traces the path that the Sun takes through the zodiacal constellations every year, and
shows the plane of the Earth’s orbit around the Sun. Since all the planets circle the
Sun in almost exactly the same plane, it also closely follows the planes of the orbits
of the other planets, too.

When Mercury is widely separated from the Sun, it is separated from it
along the line of the ecliptic. But, at different times of year, the ecliptic meets the horizon
at different angles at sunset. This means that
Mercury appears at different
altitudes above the horizon at different times of year, even if its separation from the Sun is the same.

If the ecliptic meets the horizon at a shallow angle, then Mercury has to be very widely
separated from the Sun to appear much above the horizon. Conversely, if the ecliptic is almost perpendicular
to the horizon, Mercury may appear much higher in the sky, even if it is
actually much closer to the Sun.

At sunset, the
ecliptic makes its steepest angle to the horizon at the spring equinox – in March
in the northern hemisphere, and in September in the southern hemisphere. Conversely, it meets the horizon at its
shallowest angle at the autumn equinox. Because the seasons are opposite in the northern
and southern hemispheres, a good apparition of Mercury in one hemisphere will usually
be poorly placed in the other.

At sunrise, these dates are also inverted, so that for morning apparitions of Mercury,
the ecliptic makes its steepest angle to the
horizon at the autumn equinox, and its shallowest angle to the horizon at the spring equinox.

Mercury’s elliptical orbit

A secondary effect is that
Mercury is unusual among the planets for having a significantly non-circular orbit, which
varies in its distance from the Sun by 52% between its closest approach
(perihelion, labelled P in the diagram to the right) and greatest distance (aphelion, labelled A).

This means that Mercury’s separation from the Sun at greatest elongation varies, depending where it lies
relative to the aphelion or perihelion points of its orbit. In mid-September and mid-March, the Earth is well placed
to view the long axis of Mercury’s orbit edge-on.

So, if Mercury appears in the evening sky in
mid-September, or in the morning sky in mid-March, then it appears more widely separated from the Sun
than usual. Specifically, at each apparition, Mercury reaches a separation from
the Sun of between 18 and 28°. During its Nov–Dec 2023 apparition,
it will reach a maximum separation of

21°
to the Sun’s east at greatest elongation.

The optimum time for an apparition of Mercury

The two effects described above are of similar magnitude, though the inclination of the ecliptic
to the horizon is the more
significant. They conspire to make Mercury much easier to
observe from the southern hemisphere than from the north.

In the southern hemisphere, apparitions of Mercury which occur when the ecliptic plane is favourably
inclined to the horizon also coincide with apparitions when Mercury is close to aphelion. In the northern
hemisphere, unfortunately the opposite is true: when the ecliptic plane is favourably inclined, Mercury
is close to perihelion.

The plot to the right shows the maximum altitude of Mercury during all its

evening
apparitions between 2000 and 2050,
as observed from a range of different latitudes on Earth.
The highest altitudes are seen exclusively from the southern hemisphere.

Mercury’s position

The position of Mercury when it reaches greatest elongation will be:

The coordinates above are given in J2000.0.

The sky on 4 Dec 2023

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

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Image credit

© NASA/JPL/MESSENGER