Christmas morning will also bring with it a astronomical giftin addition to those to be discarded under the tree. Mercury, the planet closest to the Sun, will in fact be at maximum west elongation on the morning of December 25th, i.e. the point of maximum angular distance in the sky from our star, equal to approximately 22nd. The exact moment of maximum angular distance will occur around 4 Italian timebut Mercury will not rise before 6 am, about an hour and 40 minutes before the Sun, being between the constellations of Ophiuchus and Scorpius, near the star Antares. This moment represents the best opportunity to observe with the naked eye this elusive planet which, given its proximity to the Sun, is always difficult to observe as it is immersed in intense sunlight.
Where and when to observe Mercury in the sky with the naked eye
Mercury will reach the maximum angular distance from the Sun, the so-called maximum elongation westequal to just over 22nd at 4 in the morning Italian time December 25th. At that time Mercury is still below the horizon in Italy, so you will have to wait until 6am to see it rise, about 1 hour and 40 minutes before the Sun.
Mercury will rise in east/south-east direction between the constellations of Ophiuchus and Scorpio, accompanied in the sky to his right by the star Antares, the brightest star of Scorpio. Mercury will be visible to the naked eyeshining with a magnitude equal to –0.4. We recommend trying to observe it shortly after it rises, since as it increases its height above the horizon, so does the brightness of the sky due to the approach of dawn. Accordingly, it is better to have a clear east/south-east horizon to be able to attempt observation immediately.
Why Mercury will be visible to the naked eye and what “elongation” means
The definition of elongation in astronomy it refers to the angle formed between two imaginary lines that connect the Earth to the Sun and the Earth to the celestial body in question. Generally, elongation is expressed in terms of angular distance between the Sun and the celestial object measured in degrees on the celestial vault.
The points of maximum and minimum elongation they represent respectively the maximum and minimum angle that the Sun and the celestial body form. When the latter is at its maximum elongation, this will also be the angular distance from the Sun and consequently this point represents the moment of best visibility of the celestial body. The planets inside the Earth’s orbit in the Solar System, Venus and Mercury, will each have a point of maximum east or west elongation, together with the minimum elongation which is called conjunction. The latter can be lower or higher depending on whether the planet is between the Earth and the Sun or on the other side of the Sun from the Earth.
When the angular distance between the Sun and the celestial body, measured from the Earth, reaches 90 or 270°, then we speak of a celestial body in quadraturewhen it reaches 180° it is called oppositionwhile at 0° we speak of conjunction. Be careful, however, that this only applies to planets that have orbits outside the Earth’s, such as Mars, Jupiter, Saturn, Uranus and Neptune.
