Russell Eberst's Sky View

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April 2018

This year Easter Sunday falls on April 1. Unlike Christmas, Easter occurs on different dates each year and, for many people, the method of deciding what date it occurs is a bit of a mystery. The rule is that Easter falls on the Sunday next following the Saturday that falls on or after the Full Moon next following the Vernal Equinox. This means that Easter can occur on any date from March 22 through to April 23. This year the equinox was on March 20, and the next Full Moon on Saturday, March 31. Since there is part of the rule that mentions a Saturday, the date of Easter this year is on the next day, April 1. The whole process of deciding the date of Easter is further complicated by the ecclesiastical authorities not using the real Full Moon but instead using what is called the Paschal Full Moon. It comes as no surprise that many people would prefer that the date of Easter was fixed, or had a simple rule such as being on the first Sunday in April, which this year would give the same resulting date. The fact that the financial year also comes to an end in early April causes problems for those trying to complete and close their accounts for 2017-2018. Let us now turn to matters celestial. The brightest planet, Venus, is steadily becoming more obvious in the evening twilight. It is still on the far side of its orbit from the Earth, but its highly reflective clouds ensure that it retains its title of "The evening star", being first to appear as the dusk twilight gradually dims. Venus is covered in thick clouds, mainly consisting of carbon dioxide. They reflect much of the sunlight falling on them, causing Venus to appear extremely bright in our skies. It is well worth comparing the steadiness of the light from Venus (a planet) with the twinkling sparkle of a star such as Sirius, the dog star, which is visible to the left of Orion.

During April, Venus moves rapidly up through the constellation of Taurus, the bull. At magnitude -4, it can even be spotted in daylight if conditions are right. Standing in the shadow of a building, and watching for a pinpoint of light in a deep blue sky, it may take some minutes for your eyes to focus on infinity and eventually spot the planet. It is helpful to use the Moon as a guide. On the evening of April 17, the thin crescent Moon is below and to the left of Venus. Indeed, that might be regarded as a "girls' night out" with the Moon (female, as in French 'la lune') together with Venus (feminine goddess) both located close to the Pleiades cluster, also known as 'The Seven Sisters'. After the Moon is near Jupiter on April 3 and 4, it again approaches Jupiter at the very end of the month, the waning gibbous disc is in the vicinity of Jupiter on the evening of April 30. At the other end of the month, on the morning of April 2, we find the two planets, Saturn and Mars in conjunction. Before dawn, looking to south-east, the two planets are 1°.3 apart. Saturn is the northern of the two and is outshone by reddish Mars at magnitude 0.2. Both are much brighter than any of the background stars of Sagittarius. The Moon is close to the planets on the mornings of April 7 and 8. Mars is rapidly approaching us and therefore becoming brighter and presenting a larger disc, with surface features becoming more easily discernible. As the upcoming close passage between Earth and Mars looms, so the optimum configuration for launching a Mars probe is reached during April and May.

Also due for launch is TESS. That acronym stands for Transiting Exoplanet Survey Satellite. Scheduled to lift-off on or soon after April 16 from the Eastern Test range at Cape Canaveral, TESS is designed to help the search for planet orbiting distant stars, often referred to as Exoplanets. Prior to 1995, we were not aware of the existence of any planets around other stars; then the star 51 Pegasi was found to have a Jupiter-sized planet circling it. A trickle of new exoplanet discoveries ensued, to rapidly turn into a torrent. Nearly 4000 of these worlds are now known, with more than 30% of that total contributed by data from the Kepler spacecraft. It was launched in March 2009 and studied star in the direction of the constellations Cygnus and Lyra. Its successor TESS has completed its testing and will be placed aboard a Falcon 9 rocket constructed by SpaceX. After a number of manoeuvres, some using the gravitational pull of the Moon, its final orbit takes 13.7 days to complete. Its distance from Earth varies from 108,000 km to 374,000 km due to its highly eccentric path. Once on station, it will monitor half-a-million stars over a two year period. It will be looking for tiny dips in the light received from each star as any exoplanet crosses the disc of the star in a transit that briefly diminishes the amount of starlight that we measure. The four cameras will seek repeated transits that will reveal the orbital period of the exoplanet. It should be possible to determine the mass, size and density of these exoplanets, and even study any atmosphere retained by the planet. The spacecraft will observe stars in one celestial hemisphere during its first year, and then switch to the other hemisphere for the second year.

Moon phases:

  • Last Quarter April 8
  • New Moon: April 16
  • First Quarter: April 22
  • Full Moon: April 30

Sky View Image

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on 30th. An arrow depicts the motion of Jupiter.