July 2, 2023

Saturday, July 2, 2023

On Thursday, July 6 at 1:06 p.m. Pacific Time, Earth will be at the aphelion point of its orbit—its farthest distance from the sun, equal to 94,506,364 miles. That’s 3.1 million miles farther out from the sun than it was in early January at the perihelion point. Since the seasonal temperature changes are caused by the angle of sunlight hitting the ground instead of the distance, July is during our summer season and January is during our winter season.

Tonight, the moon will be at full phase and it will be positioned right in the middle of the Teapot part of Sagittarius, rising about 8:30 p.m. One definition of a “supermoon” is a full phase happening at a distance equal to or greater than 90% of the distance from apogee (farthest distance from Earth) to perigee (closest distance to Earth). See world-renowned moon position expert, Fred Espenak’s webpage for all the supermoons between 2001 and 2100 using this definition.
 
Another definition of “supermoon” is a full phase happening within 24 hours of perigee. If you use the first definition, then tonight’s full moon will be one of the four super moons we’ll have in 2023. The next ones on August 1 and 30 and September 28 will be supermoons as well. If you use the second definition, then only the August 1 and 30 full moons will be supermoons. For July, the last (or third) quarter will be the night of July 9/10 when the moon will have moved to the dim stars of Pisces. 

This evening and the next seven evenings (until July 11), Mars and Venus will fit within the same field of view of your binoculars. Their closest approach to each other was on June 30. They are found low in the west after sunset and will be above the horizon until about 10:30 p.m. On the nights of July 9/10 and 10/11, Mars will be less than one degree (your thumb at arm’s length) from the brightest star of Leo, Regulus. They will be about the same brightness with Regulus very slightly brighter than Mars. You’ll probably have a hard time seeing which one is brighter but you should have an easy time seeing the difference in color with Mars an orange-red and Regulus a blue-white color. Later at night around midnight, the planet Saturn becomes visible as it rises among the dim stars of Aquarius. Jupiter follows shortly after 2 a.m. to the left of (east of) Pisces.

Sometime in early July the European Space Agency’s Euclid mission should launch (the launch date is any day after June 30, so it may have already launched). Euclid is one of three new projects that will try to figure out the nature of dark energy that is responsible for the accelerating expansion of the universe. Dark energy is a form of energy of empty space that makes up about 68% of the total universe's energy. It is responsible for making the expansion of the universe increase at a faster rate than before against the attractive force of gravity that would tend to slow the expansion rate down.

Euclid is going to survey more than one billion galaxies over about one-third of the sky. The galaxies that Euclid is going to observe are far enough away that the light we see now was emitted at the time when the universe’s expansion rate was transitioning from slowing down, due to the attractive gravity force of dark matter, to accelerating from the repulsive force of dark energy. These galaxies will have emitted their light between about three to ten billion years ago.

Euclid is going to measure the distortions of light from the galaxies caused by clumps of dark matter as well as very precisely measure their distances to create the most accurate map of the 3D distribution of galaxies in the universe. Euclid’s goal is to distinguish between three possible explanations for the dark energy. Unfortunately, the three explanations create very subtle differences in what we’d see in the past universe, so we need extremely precise and accurate measurements that can only be made in space and we need A LOT of measurements to get good enough statistics, i.e., understand how much of what we see is a true effect vs. the normal random fluctuations that happen with any process in nature. 

Euclid’s analysis will be complemented by observations from the huge ground-based Vera Rubin Observatory in Chile that will study the entire sky but not to as great a depth as Euclid. Rubin should start that study in 2025. NASA’s Nancy Grace Roman Space Telescope, scheduled to launch in 2026, will peer even further into the past than Euclid but over a narrower swath of sky than Euclid. Although any one of the three facilities (Euclid, Rubin, and Roman) could possibly solve the dark energy mystery by themself, it will take the corroboration by the other two to convince us of the truth of that solution.

To find out more about the science behind Euclid, visit the European Space Agency's Euclid webpage,  and the cosmology chapter of my online textbook. Dark matter, dark energy, dark skies—all cool stuff!

—
Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com