Mercury will tomorrow be farthest out from the Sun (to the west – the right as seen from our northern hemisphere). It’s trying to make itself visible in the pre-sunrise twilight.
But it doesn’t try very hard, and this is really only to show you why this “greatest elongation” of Mercury (the first of three morning ones in the year) is unfavorable. To see it you’ll have to try harder.
The diagram shows that although the little planet has swung out to a large angular distance from the Sun (26.7 degrees, almost the largest elongation it will reach in the whole year), it is out at a low angle to the horizon. This is because it is in the part of its orbit which slopes south from where the Sun is. (Put another way, its declination is more southerly than the Sun’s; it is farther below the celestial equator.)
For people in South Africa or Australia it’s very different: Mercury’s southerliness favors them and they will see it high in the dawn. The tilted and eccentric geometry of the innermost planet’s orbit keeps cheating us in northern lands, as has to be explained every year in the Astronomical Calendar‘s Mercury section.
For better opportunities to glimpse Mercury, we have to wait till the evenings of around May 6, and the mornings of around October 16.
I decided to switch back to drawing these diagrams for the eastern American longitude of 75 degrees west (instead of the Greenwich longitude of 0). This may help a bit in the explanation of the clock times. The absolute time of the picture, about 11 hours Universal Time, is 11 AM in Britain, 6 AM in eastern North America, 3 AM on the west coast. But the diagram is true for the time of day – the time relative to sunrise – for all of those and other longitudes. That is, the stars will be in the same places and so will Mercury, almost exactly. (Only if the fast-moving Moon were in the picture would its changing positions have to be explained.)
The instant of the Feb. 24 “greatest elongation” event is not 11 UT but 16 UT, about 5 hours after the time of our picture. But this makes no appreciable difference, because at such an event the planet is hovering essentially still in relation to the Sun. We are looking at it as it moves directly away from us – looking at it along a tangent to its orbit. So it doesn’t matter whether it’s Britain or New York or California you’re looking from about 6 AM.
Differences of latitude make a difference: if you’re farther north than 40, Mercury will be lower. If you’re at the north pole, Mercury will be below the horizon, indeed below the Sun. Reductio ad absurdum is often a trick toward understanding.