On August 10, the smallest planet in the Solar System will undergo an eastern elongation — the apparent distance between it and the Sun will be almost 27.5°. This is only half a degree less than the maximum possible value of this indicator. In fact, so much is not needed: for example, in April of this year, the elongation of Mercury did not exceed 20°, but at the same time it was quite easy to observe it in the evenings from the territory of Ukraine. However, now it will be very difficult to see it even in the southern Ukrainian regions. Let’s try to figure out why.
If you look carefully at the sky after dark, you will notice that the “low” zodiac constellations of Scorpio, Ophiuchus and Sagittarius are now in the upper climax. Moving along them, our daylight is moving far to the south of the celestial equator. Due to this, in the evening sky in autumn, the ecliptic (the conditional projection of the earth’s orbit onto the celestial sphere) is tilted more weakly to the horizon, and all celestial bodies located near it to the east of the Sun have a declination less than the solar one and hide behind the horizon shortly after it, when civil twilight has not yet ended. A celestial body in such conditions can be seen only if it has a sufficiently high visible brightness. In fact, only Venus satisfies such conditions, but now it is just entering a period of morning visibility. Even the thin crescent of the new Moon in autumn can be seen at the age of at least two days.
August is almost autumn. In addition, Mercury’s orbit is inclined to the ecliptic in such a way that during the August-September eastern elongations, it is visible to the south of it, that is, its declination is even smaller. Therefore, in our latitudes, it sets in a few minutes after the Sun. This practically eliminates the possibility of seeing it. The opposite situation is developing in the Southern Hemisphere. Do not forget that when we have autumn, spring is there, and our “low” zodiac constellations are, on the contrary, the highest there. The ecliptic turns out to be at a large angle to the horizon in the evenings, and Mercury remains in the sky in maximum elongation for almost two hours after the end of civil twilight. The August periods of evening visibility of this planet are generally the most favorable for ground-based observations, especially if they are conducted in the zone approximately between the Southern Tropic and 30° south latitude.
Such “discrimination” of the inhabitants of our latitudes takes place in the modern era, but after about 5 thousand years the situation will change markedly. Aphelion elongations of Mercury, during which its linear distance from the Sun approaches its maximum, will occur during its spring evening or autumn morning visibility periods, and the best conditions for their observations will develop in the Northern Hemisphere.
According to the laws of celestial mechanics, following the elongation, the smallest planet will undergo a standing configuration, after which it will seem to ground observers that it is moving “towards” the direction of the Sun’s movement along the ecliptic. Astrologers call such a movement “retrograde”. The current period of “retrograde Mercury” will last from August 23 to September 14, and all this time we will again have someone to blame for all our failures!
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