Mars and Moon Meet at Midnight

A double sky event will unfold in the night between Thursday July 26 and Friday July 27.

Mars is at opposition. the nearest and brightest between 2003 and 2035.  And: the Full Moon goes into total eclipse – though 15 hours later, so that it is a spectacle for lands around the Indian Ocean and, lower in the sky, for Europe, South America, and Australasia.

That these two events happen only hours apart means that they happen at essentially the same place in the sky.  Mars and the Moon meet there.

Mars is moving westward (“rightward”) against the starry background, as planets do around the time of opposition because we are overtaking them.  The Moon, with its far more rapid strides, moves in the contrary direction.  They meet near what I call the anti-Sun point.  This is the center of Earth’s shadow, which is normally invisible but now becomes visible as the Moon travels through it.

Let’s deal first with the Mars opposition, which happens first.

A planet is prominent for weeks as we pass nearest to it; the instant of opposition is just the middle of this time, when the planet is 180° from the Sun.  That instant is July 27 at 5 hours Universal Time, which is 1 a.m. for clocks in eastern North America, midnight by Central time, and back in July 26 for zones farther west.  So the scene shown above is for America on the evening of July 26, close to the opposition instant.

What is special about the current Mars opposition?

This diagram is projected on the ecliptic plane.  The gray circle is at Mars’s average distance from the Sun. about 1.52 astronomical units (Sun-Earth distances).  In red is Mars’s path for this year.  The orbit is quite elliptical (eccentricity 0.093), so its nearest-in point – perihelion –  is only 1.381 a.u. out from the Sun.

Arrows connect Earth to Mars at all oppositions from 2003 to 2020, plus the next near-perihelion opposition, in 2035.

Mars oppositions are at intervals of about 2.13 years (this planet’s synodic period).  For instance, from the 2016 opposition Mars went all around its orbit and then about another 62° before Earth’s next overtaking of it in 2018.  You can see that the jumps on the outer side of the orbit, such as from 2010 to 2012, are shorter than those on the inner side, such as 2018 to 2020, where the planet is speedier.

The quality of this year’s opposition is that it is only 51 days before Mars reaches perihelion on Sep. 16.  Near-perihelic oppositions are the best in that the planet is nearer to us, so appears larger and brighter.  They are each 7th or sometimes 8th opposition, at intervals of around 16 years.

At this year’s opposition, the distance from us to Mars is 0.385 a.u.  On 2035 Sep. 15 it will be 0.382; on 2050 Aug. 14 it will be only 0.374.  But all these are beaten by 2003 Aug. 28: less than 0.373.

2003 was the really special one.  That opposition was less than 42 hours before perihelion.  To quote Astronomical Calendar 2003:

“The chance of the two phenomena coinciding this or more closely in any 15-year cycle is, I figure shakily, about 1 in 225…  [The distance at minimum is] 0.37272 a.u. or 55,760,000 km from us.    Various calculators have announced that this is the closest we get to Mars “since the Roman empire” or since 57,537 B.C.!  Jean Meeus in his Astronomical Tables (p. 63) says that from 0 to 3000 A.D. the nearest Earth-Mars approach is 0.37200 a.u. (55,650,000 km) in 2729.”

So Mars this time will by no means break a record.  But all perihelic oppositions are good and the differences between them are hardly perceptible; what is good news is how they are in general better than the oppositions on the other side of the orbit.  In 2018 the little disk’s apparent width will be 24 seconds (as against 25 in 2003 and 2050, and 13 or 14 at aphelic oppositions).  Its brightness will be magnitude -2.8 (as against -2.9 in 2003 and 2050 and -1.2 at aphelic oppositions).

Because of the ellipticity of the orbit, the planet’s nearest moment to us does not exactly coincide with opposition (unless opposition is exactly at perihelion or aphelion).  On July 27 Mars is in the part of the orbit still curving slightly inward, so the extreme of nearness to us comes on July 31.  But it is less than 0.001 of an a.u. nearer.

So much for Mars’s nearness.  But then there’s southerliness.

This is the drawback – the literal downside – for observers in Earth’s northern hemisphere.  Perihelion is in the direction to which Earth faces outward in August.  This, being opposite to the northerly Sun of late summer, is quite far south.  It’s exacerbated by the 1.9° tilt of Mars’s orbit, also southward in our August direction, and magnified by Mars’s nearness.  Thus on Aug. 16 Mars gets as far south as declination -26°32′, well south of the southernmost ecliptic.  Throughout the time around opposition, Mars (for northern observers) rises late and doesn’t get very high and sets early.

I indicate this in the diagram with a pink mark alongside Mars’s path when its declination is below -23°.  (The southernmost point of the ecliptic has a declination of -23.44°.)

Here is a chart of the course Mars appears to take as we catch up with it and pass it.

It is well south of the equator; is also south of the ecliptic; and takes a deeper dip during its retrograde loop, when it is nearest to us.  There was also a brief episode in late March when it was south of -23° because it was south of the southernmost point of the ecliptic.

Mars will be negligibly nearer a few days after opposition, and farther north from September onward.  But the main reason you may want to turn your telescope on it a few days later is: the Moon.

As Mars comes to opposition, the Moon is marching toward it, so the sky is flooded with moonglare – until, around 10 hours after the opposition moment, the sky begins to darken as the Moon encounters Earth’s shadow.

The Moon is, at this month’s Full time, only just short of reaching its descending node across the ecliptic.  That is why it goes almost through the center of the shadow, and this will be a long total eclipse.

Here is the view from Earth to the Moon as it goes through our shadow.

The umbra is the core shadow.  No sunlight reaches it directly, but varying amounts can be refracted into it by Earth’s atmosphere.  This is why it can appear brownish or even reddish, in patterns that vary at every eclipse.  It is why the media have taken to calling a totally eclipsed Moon a “Blood Moon.”

And here is the Moon’s view of Earth passing between it and the Sun.  This shows the hemisphere of Earth from which the Moon is visible.

And here’s a timetable of what happens.  (UT is Universal Time, CDT is North American Central Daylight “Saving” Time, BST is British “Summer” Time.)

–  17:13 UT (13:13 EDT, 18:13 BST):  Beginning of penumbral eclipse: Earth’s pale outer shadow touches the Moon, which is rising for Turkey, Egypt, Congo, is in the sky east of there, setting for Kamchatka and the southern Pacific..

–  18:24 UT (14:24 EDT, 19:24 BST):  Beginning of partial eclipse: Earth’s dense inner shadow (the umbra) touches the Moon, which is rising for eastern Europe, southern Italy, Libya, Nigeria; in the sky for much of Africa and Asia and all Australasia; setting for  Hokkaido and the southern Pacific.

–  19:30 UT (15:30 EDT, 20:30 BST):  Total eclipse begins: the Moon is inside the umbra.  It is rising for southern Scandinavia, Germany, mid France, mid Spain, Morocco, the southern Atlantic; up in the sky east of there; setting for Japan, Micronesia, New Zealand.

–  20:22 UT (16:22 EDT, 21:22 BST):  Middle of the eclipse.  The Moon is in the sky for the hemisphere centered east of Madagascar.

–  21:14 UT (17:14 EDT, 22:14 BST):  End of total eclipse: the advancing edge of the Moon begins to leave the dark shadow.  It is rising for the east coast of South America and for the Atlantic off Scotland, thus is low in the southeast for all of Britain and almost all of Scandinavia; is higher for the rest of Europe, all of Africa, half of Asia, is setting for eastern China, the Philippines, central Australia.

–  22:19 UT (18:19 EDT, 21:19 BST):  End of partial eclipse: the last dark spot of Earth’s umbra quits the rear edge of the Moon, which is rising for a bit of Iceland and for Venezuela, Bolivia, Chile; in the sky for Europe, Africa, Asia as far as where it is setting for western China, Borneo, western Australia.  The inner part of the penumbra is faintly gray, the outer part indiscernible, so this is the practical end of the eclipse.

23:30 UT (19:30 EDT, 22:30 BST):  End of eclipse: Earth’s outer shadow quits the Moon, which is rising for Iceland, Newfoundland, Haiti, Colombia, Ecuador; in the sky for Europe, Africa, Asia as far as where it is setting for Bengal, a bit of Sumatra, the ocean off Australia.

In southern Britain, the Moon rises about 19:40 UT, which is 20:40 BST.  So the total eclipse has just begun.

The Full Moon, besides being near its descending node, happens to be near another geometric marker, its apogee, the outermost point of its orbit, so it is more distant and smaller than average, but this is not noticeable.  In fact you will probably think it looks larger than usual.  But this is because you are seeing it low to the horizon: the notorious Moon Illusion.  (I call it “notorious” because it is so strong that many people cannot be convinced that it is an illusion.)

The Moon passes due north of Mars almost exactly at the moment of mid eclipse.  There is Mars, about 6° to the Moon’s south – but it’s on the horizon,  To see it you’d have to be in a more easterly country where the eclipse is higher, or to wait till after the eclipse when the sky has re-brightened and Moon and Mars are higher.

And here’s a confession.  Lorraine, a sharp-eyed reader, gently pointed out that my “Astronomical Calendar 2018” says, at the same date as for the eclipse:

“Moon 6.6° N. of Mars , only about 3° from the Sun .”

How could that be?  Of course it cannot.

It took me a while to discover the bug.  One of the things done by my program for finding conjunctions of bodies is that it finds the average of their elongations (angular distances from the Sun) so as to be able to say that the event is close to the Sun or is “in the evening sky” or “the midnight sky” or “the morning sky.”  But in this instance the elongation of the Moon is +180 (rounded to the nearest degree) and that of Mars is -173.  The average of those is about 3, whereas what we want is 176!  So another tweak is needed in the program.

I opted not to take time to correct the online page, because of the trouble it would take, and so as to leave this as a mildly interesting monumentum erroris.