Total Lunar Eclipse – July 27^th 2018
Guest Blog by John Flannery of the Irish Astronomical Society

The evening of Friday, July 27^th, sees the longest total lunar eclipse of the 21^st century visible over a wide area of the Earth. From Ireland totality is already in progress at moonrise (9:22pm from Dublin) and with the sky still relatively twilit, it may initially be a challenge see the reddened lunar disk. The Moon will rise towards the southeast so a low horizon is a prerequisite to see it, and is only about 5° up (just under a fist-width) when totality ends at 10:13pm. Following that the Earth’s shadow will slide off the Moon’s face over the next hour or so as we see the latter partial stages of the eclipse play out. Binoculars will enhance the view — especially if the Moon is an indistinct orb as it rises.

What is a lunar eclipse?

A total lunar eclipse occurs when the Moon passes through the shadow of the Earth cast into space. This means that we can only get a lunar eclipse during time of Full Moon when the Sun, Earth and Moon are in a straight line. We don’t get a solar or lunar eclipse every month however because the orbit of the Moon about the Earth is tilted with respect to the Earth’s equator.

During a total lunar eclipse our atmosphere acts like a lens bending sunlight around the Earth’s limb onto the Moon. Longer wavelengths of light (red and orange) penetrate our atmosphere better than shorter (blue) so during totality the Moon takes on a reddish-orange hue. The effect is similar to the reddening of the setting Sun. It once led someone to comment that the red colour of the eclipsed Moon is due to all the sunrises and sunsets around the world being painted on the Moon — a rather nice and perceptive observation.

Mars comes near

The planet Mars rises about 50 minutes after the Moon and appears as a strikingly bright yellow-orange “star” a little below our satellite. It is at opposition — when Mars, the Earth, and the Sun, are almost in line — the same night as the lunar eclipse. This year the planet is closer to us than any time since 2003. It even outshines Jupiter at the moment and does so until early September.

A telescope is necessary to make out any details of the Martian surface but an almost planet-wide dust storm which brewed up in May has hobbled the efforts of professional and amateur astronomers. The storm is slowly clearing now though but whether it has affected spacecraft currently operating on the planet’s surface is still to be determined. On its way to the Red Planet at present is NASA’s Insight spacecraft which will land on November 26^th to probe the interior of Mars. Spacecraft are launched during periods when the Earth-Mars distance is at a minimum.

Other planets

Besides Mars, the planet Venus is a brilliant lamp low in the western sky after sunset these evenings while Jupiter is a cream-coloured bright “star” in the southwest. Binoculars will let you pick out Jupiter’s four largest moons which change position from night to night as they circle their parent. Almost due south a short distance above the horizon you’ll find yellowish Saturn snug within the summer Milky Way. A telescope is necessary to get a decent view of Saturn’s rings and for the first time observer the sight is a real thrill.

Other sights

The International Space Station passes over Dublin at 11:05pm on July 27^th. Predictions for other sightings from your location can be generated from www.heavens-above.com.

By John Flannery, Irish Astronomical Society.