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There are a number of theories as to how our moon formed, or where it may have come from.
The primary one - which is backed by most lunar scientists - is called the "giant impact hypothesis." Its premise is that the moon formed as a result of a collision, approximately 4.5 billion years ago, between the newly-formed Earth and a Mars-sized object (given the name "Theia"), with the moon forming when some of the debris coalesced around the core of Theia.
A variation of this theory has the moon forming from a number of massive objects that struck the young planet, sending up large quantities of material that eventually formed the moon.
Another variation, put forward in 2012, theorizes that the moon formed when two massive objects, five times the size of Mars, crashed into one another, with the Earth created from the majority of the collision material and the moon from the remaining debris disk that orbited the new planet.
The second theory, referred to as the "co-formation theory", holds that the moon formed much as (and at the same time as) the Earth. It postulates that as the dust and gas in the giant molecular cloud that would eventually form our solar system coalesced into the sun and the various planets, some of that material also formed the moon. This theory provided a plausible explanation as to why the Earth and moon are very similar in material-makeup.
The final theory, called the "capture theory", posits that the moon is a celestial object, formed elsewhere in the solar system, that was captured by Earth's gravity. One flaw in this theory, however, is that such objects are usually oddly-shaped (e.g., some of the moons of Saturn), not spherical like our moon. A variation of the capture theory states that Earth's gravity, at some point billions of years ago, stole the moon from Venus (which has no moon).
This week's sky
Mercury remains too close to the sun to be seen at present.
Jupiter (magnitude -2.7) will be at opposition (directly opposite the sun as seen from Earth) on July 14. It will also be at perigee (closest approach to Earth) and will be at its biggest and brightest for the year. Binoculars will show the four Jovian moons - Io, Callisto, Europa and Ganymede - orbiting the planet. Jupiter will become visible above the southeastern horizon shortly before 10 p.m., remaining visible in the night sky (highest in the southeast at midnight) until around 4:40 a.m., when it will be lost to the approaching dawn.
Saturn (magnitude +0.17) is visible about 10 degrees above the southeastern horizon around 10:30 p.m., reaching 23 degrees above the southern horizon around 1:50 a.m., before succumbing to the dawn twilight in the southwest shortly before 5 a.m.
Mars (magnitude -0.64) is visible about 41 degrees above the southeastern horizon shortly after midnight, before fading from view shortly after 5 a.m.
Our "morning star" Venus (magnitude -4.48) shines brilliantly above the eastern horizon around 3 a.m., reaching 20 degrees above the horizon before fading from sight as dawn breaks shortly after 5 a.m.
Try and catch a glimpse of Comet C/2020 F3 NEOWISE in the constellation of Lynx in the northwest evening sky around July 15. Look to the lower right of the front feet of Ursa Major - the Great Bear, just as the sky begins to darken between 9-10 p.m. By July 23, it will be visible at the same time of night closer to the bowl of the Big Dipper asterism in Ursa Major.
Discovered in March 2020, it has now developed two tails (one gas, one dust). The comet passed perihelion on July 3 and will make its closest approach to Earth (perigee) on July 23.
Though now fading (currently estimated at magnitude +0.1), it may be visible to the naked eye under a clear, dark sky throughout the remainder of the month, with binoculars and small scopes enhancing the view; a timed photo shoot should capture the comet nicely. Google the comet online for the latest updates and finder charts.
Until next week, clear skies.
July 14 - Jupiter at opposition/perigee
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at email@example.com.