Dr. Rishi Shah

The night sky during this month would present optimum opportunities to observe and marvel at the magnificence of planets, stars and constellations along with other numerous celestial entities. The elusive planet Mercury will reach its greatest eastern elongation (sparsely 26.8 degrees) from the Sun on 14 September. Although it would be staying at its highest point above the western horizon after sunset, appreciating it would be doubtfully difficult. Planet Venus can be viewed after sundown in the southwestern sky.

The ruddy planet Mars will be lost in solar glare this month due to its proximity to the Sun. Mercury and Mars would be drifting across the western part of the constellation Virgo (maiden). Venus will be gliding across Virgo first and then through the compact constellation Libra (scales) from the middle of the month.

Generally, Mercury would be appraised in the twilight. Mercury's orbit would lie closer to the Sun than that of the earth. It would be conspicuous in the sky for few weeks at a substantial separation alias elongation from the Sun. These apparitions would repeat roughly once every three to four months alternately during morning and evening. Mercury would be the most petite planet with a diameter of fairly 4880 kilometres. It will fleet around the sun in simply 87.97 earth days.

Phases of Mercury
The planet would telescopically display the complete range of phases, similar to that of Venus and the moon. Mercury's axis would express the smallest inclination and its orbital eccentricity would be the largest in the Solar System. At Perihelion (nearest place to Sun) it would be forty-six million kilometres away. At Aphelion (the farthest site from Sun) it would be 69.817 million kilometres away.

Mercury's topology has been heavily cratered. Mercury has been tidally locked with the Sun. It would rotate exactly three times for every two revolutions it would undertake around the Sun. Without the atmosphere to retain heat, temperatures on Mercury would alter acutely from −173 degrees Celsius at night to 427 degrees Celsius during the day. The planet is devoid of natural satellites. NASA’s two spacecraft Mariner-10 and MESSENGER have studied Mercury. The European Space Agency (ESA)'s and Japan Aerospace Exploration Agency (JAXA)’s BepiColombo mission would arrive ambitiously at Mercury in 2025.

The mighty planet Jupiter could be applauded well in the southeastern sky from night-begin till after midnight above the western sky. It would be gleaming glamorously with the stars describing the eastern domain of capricious constellation Capricornus (sea goat). The ringed planet Saturn could be admired in the southeastern sky after dusk till midnight in the south-eastern sky. It would be glistening with the stars that have been dwelling in the eastern district of Capricornus.

The far-flung planet Uranus could be seen serenely from a late night in the southeastern sky until early dawn before sunup in the southwestern sky. It could be spotted at the relatively barren expanse of the sky unfurling in the southern section of puny constellation Aries (ram). The charismatic constellation Cetus (Sea Monster) would be sprawling splendidly below Uranus with its giant red binary variable star Mira, which would be modestly three hundred light-years away. Planet Neptune can be perceived after nightfall in the eastern sky until day-break in the southern sky. On 14 September far-away giant planet Neptune will be in opposition to the Sun. Furthermore, it would be opposite to earth and additionally make its closest approach to our planet. Its face would be fully illuminated by the Sun and be brighter than any other time of the year and will be visible all night long.

This could possibly be the best time to look at and photograph Neptune. Due to its extreme distance from earth, it would appear as a tiny blue dot in the eastern area of the charming constellation Aquarius (water bearer) even through powerful telescopes.

The confounding Circlet asterism (imaginary starry figure) belonging to the congenial constellation Pisces (fishes) would be hovering above Neptune, which would be whooping 4.33 billion kilometres away from earth.
The new moon (transpiring on fathers’ day) would befall on 07 September, while the full moon would mystify moon-gazers on 20 September. Its popular nomenclature of full corn or harvest moon would signify corn being harvested during this time of the year. Coincidentally, it would be mesmerising the full moon-enthusiasts in the neighbourhood of the September equinox each year. When the moon would enter the fascinating full moon phase, it would be circa 389 thousand kilometres away from earth.

It would be located in the constellation Pisces. Local famed festivals of Teej especially for women, Rishi Panchami and Indra Jatra would be celebrated joyfully on 09, 11 and 19 September 2021 respectively. The September equinox would occur on 23 September. The Sun would shine directly on the equator and almost equal amounts (twelve hours) of day and night throughout the world per se could be experienced.
This day would herald the first day of fall (autumnal equinox) in the Northern Hemisphere and the first day of spring (vernal equinox) in the Southern Hemisphere.
Seasons along with equinoxes and solstices (varying longest day and shortest night and vice versa in the two geographical hemispheres of the earth) happen because the axis of the earth-spin has been tilted at paltry 23.5 degrees to the plane of its orbit around the Sun. The earth would revolve around the Sun once every 365.242 days. The word equinox has been derived distinctly from the Latin words aequus (equal) and nox (night).

Asteroid 89 Julia
This month a large asteroid 89 Julia could be chased from evening twilight in the southeastern sky till midnight in the south-western sky. This crumpled ball-like asteroid would be mingling with stars inhabiting the north-western acreage of Aquarius. The resplendent red supergiant star Enif in the great square-depicting constellation of Pegasus (winged horse) would be floating fancily above Julia.
The traditional name Enif has its origin from the Arabic word for nose, due to its position as the muzzle of Pegasus. It is estimated to be solely 690 light-years away. Julia has been identified by French astronomer Edouard Stephan in 1866. Its diameter would be merely 151.4 kilometres. Its orbital period would indicate barely 4.07 years.

The iconic comet 67P/Churyumov–Gerasimenko could be followed through telescopes from late wee hours before midnight in the eastern sky till morning in the south-western sky. It would be hurtling through constellations Aries and Taurus (bull) with a velocity of thirty-eight kilometres per second.

With its orbital and rotational period of 6.45 years and 12.4 hours, the longest and widest dimension of its rubber duck-shaped nucleus would be purely 4.3 and 4.1 kilometres. Churyumov–Gerasimenko had been confirmed in 1969 by Soviet astronomers Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, after whom it has been tagged. The European Space Agency (ESA)'s Rosetta craft had rendezvoused with Churyumov–Gerasimenko in August 2014. Rosetta's Philae had landed on the comet's surface, becoming the first spacecraft to settle on a comet’s body. Rosetta mission ended in September 2016.

The peculiarly arcane asteroid, dubbed 2021 PH27 with the second-shortest route of meagre 113 days in the entire Solar System after that of Mercury has been recently detected. The elliptical trail around Sun would carry it across the trajectories of both Venus and Mercury. With one kilometre across it would swing extremely close to the sun to feel the scorching temperatures up to 480 degrees Celsius.
Within perhaps one million years, it would either be hurled away from its current circuit or be destroyed in a collision with one of the two planets or the Sun. It could have been dislodged by gravitational forces from the main asteroid belt between Jupiter and Mars or could have been an extinct comet that had ventured too far into our inner Solar System.

However, 2021 PH27 could still divulge diverse information about the evolution of our planetary system. Ascertaining asteroids inside Earth's course (known as Atira asteroids) would be painstaking. They will be travelling in Sun's vicinity to be scrutinised in solar irradiances. The best time for recognising them would be in the case of Mercury or even Venus just before sunrise or after sunset when the asteroid would be amply lit up but not outshone by the Sun.

(Dr. Shah, is an academician at NAST and Patron, Nepal Astronomical Society (NASO).