Use mobile apps to explore Pluto in opposition

This week the small and distant dwarf planet Pluto will reach the opposition, the time of year when the object is closest to the Earth and the brightest in the sky. As a happy coincidence, the moon will be close to its new phase, leaving our sky very dark to chase the difficult dwarf planet into large telescopes. In this edition of Mobile Astronomy, we will show you how to find Pluto and maybe even see it for yourself. We will also take you to Pluto and watch the Earth and the moon cross the sun's disk in a rare planetary alignment.

Pluto to Opposition

Pluto takes 248 years to orbit the sun. But once a terrestrial year, the faster movement of our planet transports us between the sun and Pluto, a phenomenon that astronomers call opposition.

When a planet is in opposition, it rises at sunset and spends the night crossing the night sky. This is the best time to observe the planet, as it will always shine at its strongest and display a fully illuminated face with the largest apparent disk diameter seen from Earth during the year. This is because the distance between the Earth and the planet is minimal. (Everyone is looking forward to the opposition of March later this month!) [Pluto and Charon Shine in Most Detailed Topographical Maps Ever]

On Thursday, July 12, Pluto will reach the opposition by 2018. By that time, Pluto will be the closest of the Earth (3.03 billion miles, 4.87 billion kilometers, or 271 light-minutes) and brighter (visual magnitude +14.2) for 2018. Because the moon will be close to its new lunar phase, and out of the evening sky, the conditions will be optimal to spot this difficult target in the telescopes.

The dwarf planet will be close to its best for several days on either side of July 12, so do not worry if you have a cloudy night night sky.

Finding Pluto in the Sky

Pluto is small and distant, which makes it difficult to see without the equipment and conditions of the sky . Its diameter is only 1,473 miles (2370 km), only 19% of the Earth's diameter. The distance between the dwarf planet and the sun varies between 29.6 and 49.4 AU (an astronomical unit is the average Sun-Earth separation), so Pluto varies in brightness between 16.3 and 13.7 (the numbers of magnitude increase when the luminosity levels of objects decrease)). This year, Pluto will display a brightness of magnitude 14.2. The dwarf planet will not reach its maximum brightness from 13.7 up to the 23rd century!

U.S. Astronomer Clyde Tombaugh discovered Pluto on photographic plates taken on January 23 and January 29, 1930. He chose this interval of one week between photographs to allow the planet to move relative to the stars background – but not too much, which could have been hard to follow. At the time, Pluto was in the winter constellation of Gemini. The object was 42.3 AU from the ground and shone at a scant 15.3 magnitude. Fortunately, it was slightly outside the concentrated star fields of the Milky Way of Winter, and as a result, the photographs were not too crowded with stars. Tombaugh programmed the footage for when the moon was out of the evening sky, too.

Tombaugh then loaded the pair of photographs into a flashing comparator machine that allowed him to quickly turn between them, and he noticed an object that had changed location. After further follow-up to confirm the orbit, the first planet discovered by an American was announced.

You can recreate Tombaugh's discovery by selecting and centering Pluto in your astronomy application, and then changing the date to the evening of January 23, 1930. The brilliant Wasat star (Delta Geminorum) will sit 36 minutes arc (about the diameter of a full moon) above Pluto. Zoom into the display until Pluto and Wasat are separated, then take a screen cap. Now change the date to January 29 and take another screen cap. By comparing the two screens, you can see how Pluto moved to Wasat. That's what Clyde Tombaugh saw.

Alternatively, you can select and center on Wasat and take another pair of images. Or, you can start on January 23 and move forward one day at a time – watch Pluto cross the stars.

The evenings around the opposition in 2018, Pluto will rise east at sunset and reach its highest altitude on the southern horizon at 1:30 local time. Coincidentally, Pluto will once again be near the edge of the Milky Way, adding to the challenge of spotting the dwarf planet. However, the new moon will provide perfect conditions for hunting.

To find where Pluto is at the end of the evening, go where you have a southern horizon clear of trees and buildings and look for Mars very reddish (it will be hard to miss all month). Then, find yellowish, but dim down Saturn sitting at about 33 degrees (or three diameters of fist when held at arm's length) at the top right of Mars. Using your app, you can select and center Pluto, then activate the compbad mode of the app, by moving your device against the sky and noting where Pluto is. (It's a good idea to first test the accuracy of your compbad and gyroscope on Mars or Saturn.)

To find Pluto without your device, once the sky is dark enough and the stars are starting to appear, look for teapot-shaped asterism by the brightest stars of Sagittarius. Kaus Borealis, the star that marks the pointed lid of the teapot, will have 2.5 degrees (or the width of a few fingers) below Saturn. The pot handle will be at the bottom left of this star, while the beak will be found at the bottom right. The teapot will be very obvious once you find it.

Pluto will be located 7.5 degrees (about the width of a palm) to the left of the handle of the teapot, halfway between Mars and Saturn, but several degrees above the line imaginary that connects these two luminous planets. The star at the nearest naked eye will be Al Baldah (Pi Sagittarii), which will be positioned at 3.75 degrees from the upper right of Pluto. [A Brief History of Pluto Viewing: From Its Discovery to New Horizons Flyby]

Using binoculars, explore the sky at the bottom left of Al Baldah to find the designated 5.55 magnitude star 50 Sagittarii. (The star will be bright in the binoculars.) Pluto will be positioned only 12.5 minutes west (right) of this star. If you have access to a 10-inch (254 mm) telescope (or more) and a very clear sky, try Pluto. The star and the dwarf planet will both be inserted into the field of view of the medium-high power telescope. Compare the star field you see in the eyepiece with the stars displayed near Pluto in your application.

Remember that your telescope can reverse and / or reflect the sky. In the SkySafari 5 app for Android and the SkySafari 6 iOS app, tap the upper right corner of the screen to display the toggle buttons and use the combination matching your telescope. (It's a good idea to determine the orientation in advance using a larger and brighter object like Saturn.)

Even though Pluto is too weak to see in your telescope, you will now have the satisfaction of knowing that the dwarf planet is there, just beyond the edge of our family of planets.

Visualize a transit of the Earth on the sun since Pluto

This opposition of Pluto will also produce a rare and interesting phenomenon that you can model in the application SkySafari: the transit of the Earth and the moon through the sun, seen from Pluto!

For the most part, all the main bodies of our solar system have orbits aligned at several degrees to the equatorial plane of the sun – and one to the other. This means that they can often form alignments. The inner planets Mercury and Venus can cross the disk of the sun seen from the Earth; the moon can pbad in front of the sun, producing a solar eclipse, or pbad in front of or obscure another planet. Very rarely, one planet can hide another, seen from the Earth.

However, objects in the Kuiper Belt, or trans-Neptunian objects – including Pluto – have orbits very different from those of the eight clbadical planets. The orbit of the dwarf planet is tilted 17 degrees from the equatorial plane of the sun, one of the reasons why the International Astronomical Union (IAU) has demoted the object to the status of dwarf planet in 2006.

This strongly inclined orbit usually prevents alignments with other major objects. But when Pluto is in opposition this year, the Earth will fall on the imaginary line connecting the dwarf planet to the sun. And at that moment, an observer on Pluto with a powerful telescope would see the tiny black disks of the Earth and the moon crossing the surface of the sun! We do not know any Pluto astronomers, but we can simulate the show using the SkySafari application.

Select Pluto, tap the Orbit icon, and the app will fly you to Pluto. Then, search for Earth and press the Center icon. The view will move to show the sun and the planets. Use two fingers to enlarge the sun until the Earth appears. (If you lose sight of our planet, press Center again.)

Open time flow controls by pressing the Time icon on the toolbar, then set the date to July 12th 2018 and the hour on 5:30. Summer time (adjust for your own time zone). If you continue to enlarge the display, the sun will fill the screen and the round and black silhouette of the Earth will appear on the sun's disc. Advance the hour from hour to hour to watch the Earth cross the sun disk. This transit will take about 10 hours. If you enlarge the display enough, the moon disk will also appear; notice that the orbit of the moon around the Earth brings it closer to the planet while the pair of objects crosses the sun.

For fun, zoom out and repeat the exercise using other planets. Some planets will force you to walk day by day, while outer planets will require you to vary the month. None of them will even happen to touch the sun when they pbad it.

In the next edition of Mobile Astronomy, we will see the next close opposition of Mars. In the meantime, keep looking!

Editor's note: Chris Vaughan is a specialist in astronomy and public education at AstroGeo, a member of the Royal Astronomical Society of Canada and an 74 telescope operator from the David Dunlap Observatory (1.88 meters). You can reach him by email or follow him on Twitter @astrogeoguy as well as on Facebook and Tumblr

This article was provided by Simulation Curriculum, the leader in space science course solutions and the creators of the SkySafari app for Android and iOS. Follow SkySafari on Twitter @SkySafariAstro . Follow us on @Spacedotcom Facebook and Google+. Original article on Space.com