- if Mercury is closer to the northern hemisphere summer solstice, its maximum elongation is only around 18˚ or 19˚
- it Mercury is closer to the northern hemisphere winter solstice, its maximum elongation is as much as 25˚ to almost 28˚
- the mean for all elongations is 22.6˚ for both western and eastern elongations, but western ones show a larger variation than eastern
|Elongations of Mercury from 2000 to 2015 (“western” positive; “eastern” negative)|
|Sky map (drawn from ‘Your Sky’) for Fairbanks, Alaska, 21:50, 15 August 2016. Note the extremely narrow angle between horizon and ecliptic plane, and that Mercury, though at maximum eastern elongation, is setting with the Sun and is hence not visible|
Consequently, at these farthest elongations, a skywatcher in Arctic or subarctic regions will never get Mercury at all visible: the Sun will completely block any attempt to see the planet.
The same principle applies to Mercury’s farthest western elongations, which occur with the Sun and Mercury are inverted in name, and their positions flipped by 180 degrees of ecliptic longitude. In astrological terms, one reverses the “planets”’ names and then places them in opposite signs. Be careful that doing one or the other would be impossible since Mercury would move from aphelion to perihelion and could never be at so large an angular distance from the Sun when viewed from Earth.
If we go beyond the polar circle the problem becomes even more extreme and interesting. Taking Dikson, Krasnoyarsk Krai – before virtually emptying the northernmost town over 10,000 in the world – we can see something quite interesting occur on 7 to 8 April (entirely 7 April 1993 UTC) from the perspective of a horoscope:
As you can see, Mercury is in either picture never above the horizon at any point where the Sun is below. Indeed, as confirmed by the charts for 2300 and 2348 UTC earlier in this post, and from the royal blue picture that is used in ‘Your Sky’ to indicate that the sky is bright, Mercury is below the horizon of a fully brightened sky all along.
(For the curious and for those who would want to examine ‘Your Sky’ further, dark blue indicates astronomical twilight, dark red civil twilight, and black is completely dark night).
If we look at the near elongations that occur when Mercury is closer to the northern hemisphere summer solstice than is the Sun, we of course have the problem that Mercury cannot be as far from the Sun as it could potentially be at other elongations, including those where at polar latitudes both planets are necessarily above or necessarily below the horizon. However, it is in Arctic latitudes that one might expect the possibility that a near elongation could provide a reasonable view of Mercury. Thus, we will look at the near elongation following the August 2016 far elongation. For Dikson, this occurs on 27 September 2016 at 23:09 UTC (05:09 28 September 2016 local time):
|Sky view for a “favourable” Mercury elongation on 28 September 2016 at Dikson|
As we can see, even here the viewing of Mercury is not very favourable – and such near elongations which allow even this good a view of Mercury from the Arctic are very rare because they must occur on a late September or early October morning. We can note Mercury just above the horizon at its maximum elongation of 17.9 degrees, but the planet does not get more than 12 degrees above the horizon at sunset during any period of 28 September. More than that, in the ‘Your Sky’ diagrams for Dikson on this day, there are substantial periods when Mercury is no shown even with the Sun below the horizon and the planet above.
Unless the guides are inaccurate, this does suggest that Mercury is almost impossible to view in Arctic skies. The horoscope diagram shows that from the point of view of the Prime Vertical, the angles even at sidereal times when the period of the “Midnight Sun” is on the Midheaven are not sharp enough for easy viewing:
- the angle at far elongations is extremely low or even negative so Mercury is only above the horizon of a bright sky
- at solstitial elongations the Sun and Mercury will never (or barely at subarctic latitudes) cross the horizon
- at steeper-angled elongations on autumn mornings (or spring evenings, not shown) Mercury is only about 18 degrees from the Sun and can never get more than 6˚ above the horizon of a sky before civil dawn or after civil dusk
|Sky at Vostok, Antarctica for the 27.8-degree western Mercury elongation of April 1993|
|Sky at Vostok, Antarctica, for the 27.4 degree eastern elongation of Mercury in August 2016|