July is a perfect month to make a little getaway to the countryside and enjoy a night of astronomical observation . Although it is true that we will not have any great event such as the eclipse of last May, the planetary conjunctions of March and April or the shower of stars in August, we will be able to enjoy the simple spectacle of a starry sky and the many stars that populate it.
This month a phenomenon will occur that usually goes unnoticed, but that is one more example of the complexity and interest of our firmament. The star Mira , which is part of the little-known constellation of Cetus (or the constellation of the whale) is a variable red giant star . That is, due to the expansion and contraction suffered by its outermost layers, its brightness varies over time and it does so following a well-defined periodicity. Specifically, Mira takes about 332 days to complete one of its cycles, during which it goes from an apparent magnitude of 10 (that is, invisible to the naked eye and only detectable with a powerful telescope and good atmospheric conditions) to a magnitude of 2 (that is, say, as bright as Saturn or Mars at their dimmest).
This star was already known by the Greeks and Romans, who saw how, approximately every year, this star appeared and faded while its brightness varied . This was what gave her her current name, Mira, which would come from Latin, meaning “wonderful” or “amazing.” Well, the maximum brightness will be reached this year on July 16 . We will be able to see it at dawn, towards the southeast and located close to Mars and below the red planet.
This maximum in brightness of Mira will almost coincide with the lunar maximum brightness, with the full moon , of this month, which will occur on July 13 shortly before sunset. The moon’s brightness will decrease until the new moon on July 28 . This full moon will be one of the so-called supermoons , which fill the headlines of digital media today. This supermoon occurs when the moment of maximum brightness of the moon (the full moon) coincides with its moment of closest approach to Earth (the perigee). This coincidence gives us a slightly larger full moon than when these two moments do not coincide, although the difference is so subtle that it is barely noticeable with the naked eye . It is perhaps the task of those of us who dedicate ourselves to astronomical dissemination and to writing the headlines of these news items to place the viewer’s expectations where they belong: it will be a big full moon, yes, but nothing fanciful or epic.
Also this month of July we have a day of marked importance due to the distances at which two stars are located: the Earth and the Sun , which will reach their maximum annual distance on July 4 , as happens every year. This point in the Earth’s orbit is known as aphelion and it is just that: the point where the Earth reaches its maximum distance from the Sun and from which it begins its path back to the star. This point exists because the planet’s orbit around the Sun is not perfectly circular, but rather elliptical . This will imply that there will be a minimum distance, which is reached around January 4, and a maximum, which is reached on these dates. Of course the difference between these two distances is so small that it barely influences the course of the seasons . Although we are located further away than normal, July will continue to be a hot month in the northern hemisphere, as it belongs to summer.
During this month we can enjoy the typical summer constellations, such as the Lyre, the Swan or the Eagle , dominated respectively by the stars Vega, Deneb and Altair , which form the famous asterism of the Summer Triangle . An asterism is any set of stars that form a recognizable pattern in the sky. This is not the same as a constellation because the latter are defined regions of the sky, recognized by the different Astronomical Unions and standardized internationally. That is, anyone can “establish” or “create” an asterism, pointing to a group of stars that form a certain pattern, but not everyone can establish a constellation, which is regulated by astronomical authorities.
Also other constellations, present the rest of the year, can be seen with particular ease (because of the weather rather than the conditions of the sky itself) such as the Great Bear and Minor Bear, Cassiopeia, Cepheus or the Dragon . Also Scorpio , with the bright red Antares and the trident of stars that seem to come out of it, or Sagittarius , with its characteristic teapot shape, are easily recognizable during these dates. It is in the direction of Sagittarius where the galactic center is located , so in this area of the sky, if light pollution allows it, we can observe a greater concentration of stars and dust in the band that forms the Milky Way. Behind that dust and 27,000 light-years away is the recently imaged supermassive black hole Sagittarius A* . Of course at first glance we can not even begin to dream of seeing it.
