Gateway to the Stars April 2016

Nick Jarvis

This month in Gateway to the Stars, we give a fond farewell to the bright constellations of Winter. As Orion, Gemini, and Taurus begin to drift out of view, we can turn our attention to the emerging Spring and Summer constellations and look at some interesting astronomical events this season.

In particular, the planets have some interesting things in store for the coming weeks.


Spotting Mercury can be tricky. Because the planet is so close to the Sun, looking towards Mercury means we’re also looking towards the Sun’s glare, and you know what that means: put on those solar glasses! The best time to see Mercury is when it has the biggest separation from the Sun, which is called the maximum elongation. Mercury will reach this point on April 18th, and around that date it will be visible for a short time in the evening just after sunset.

After it passes maximum elongation, Mercury will swing around to the same side of the Sun as Earth. Then, next month, the planet will line up just right to pass in front of the Sun! In astronomical terms, this event is called a “transit,” and this will occur on the morning of May 9th. The last time this happened was 2006, and after this transit we’ll have another in November 2019. After 2019, we won’t see this again until 2039! (Actually, there will be a Mercury transit in 2032, but that one won’t be visible from North America. Phooey!)

Image credit: Eric and Holli

In this month’s show, we’ll use our virtual model of the solar system to find Mercury in the evening sky, and also to simulate the appearance of the Sun as Mercury moves in front of it.


Mars will also be putting on a show for us this Spring. Currently, Mars can be found in the pre-dawn hours, near the constellation of the scorpion to the south, and is distinguished by its red-orange color. Mars doesn’t stay in one place; all of the planets will gradually drift through the constellations as they move. Most of the time this movement is a steady march from West to East, but every so often a planet will appear to change its mind and go backwards for a few weeks. This phenomenon is called “apparent retrograde motion??,” and this was a big puzzle for ancient astronomers who thought the Earth was the center of the universe. Mars will start moving backward on April 17th, and will continue in this pattern until June 29th. The reason this happens is that Earth is moving, too. Even for us in the 21st century this can be hard to visualize, but that’s what planetariums are for! As we look at Mars’ motion we’ll show how the combination of Earth’s and Mars’ orbits around the Sun create this effect.

Retrograde motion happens when both planets are on the same side of the Sun, which also means that they get closer together. The precise moment when Mars is nearest to us is called “opposition?,” and this will occur on May 22nd. In the coming weeks as we approach this point, Mars will gradually appear brighter and brighter in the sky and will appear much larger in a telescope.

For many years now, Mars oppositions have been the subject of perennial e-mail chain letter hoaxes, often claiming that “Mars will be as big as the Moon!!!” While Mars will look very pretty, it will still be much too small to be anything but a bright point of light.

Apparent sizes of Mars around its closest approach. Image credit:

Apparent sizes of Mars around its closest approach. Image credit:

In addition to watching the dance of the planets, we’ll also look at the upcoming Lyrid meteor shower which will peak around April 22nd, we’ll have a round-up of the deep sky objects visible at this time of year, and we’ll discuss the equipment and techniques you can use to get the best views of all these things going on in the sky.

As always, this show is a great opportunity to get your questions answered and to get advice on stargazing, amateur astronomy, and anything else space-related that you’ve ever wondered about.

Hope to see you there!

Gateway to the Stars? is hosted by Nick Jarvis on Saturday, April 2nd at 6:45pm in the Hansen Dome Theater. Tickets are $2, or free for planetarium members. Buy tickets here, or at the Clark Planetarium ticket desk.?

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