Skynotes: July 2023
Upcoming events
It’s aphelion time
In its elliptical orbit Earth reaches its annual furthest point from the sun this month, known as aphelion, when we will be 5 million km more distant than our closest point which occurred in January. See Planets section for more details
Melbourne Sun times
| Date | Rise | Set | Day length | Solar noon§ |
|---|---|---|---|---|
| Saturday 1st | 7:36am | 5:11pm | 9:34 hours | 12:23pm |
| Tuesday 11th | 7:34am | 5:16pm | 9:42 hours | 12:25pm |
| Friday 21st | 7:26am | 5:23pm | 9:54 hours | 12:26pm |
| Monday 31st | 7:21am | 5:31pm | 10:09 hours | 12:26pm |
§ When the sun is at its highest, crossing the meridian or local longitude.
Moon phases
| Phase | Date |
|---|---|
| Full Moon | Monday 3rd |
| Third Quarter | Monday 10th |
| New Moon | Tuesday 18th |
| First Quarter | Wednesday 26th |
Moon distances
Lunar perigee (closest to Earth) is on Wednesday 5th at 360,149 km.
Lunar apogee (furthest from Earth) is on Thursday 20th at 406,289 km.
Planets
Mercury is not visible early in the month being too close to the sun after having moved behind the sun through superior conjunction, but by the end of July it will be visible again but at dusk low in the north-west from 6pm before setting by 7.30pm.
Venus continues as ‘the evening star’ in the north-west visible from 5.3pm and setting by 8pm. It has just passed its greatest elongation east, its furthest angular separation from the sun as seen from Earth.
Earth reaches aphelion (furthest from the Sun) on Friday, July 7th at a distance of 152,093,251 km. By comparison, perihelion (closest to the Sun) for 2023 was on January 5th at 147,098,925 km, some 5 million km closer. Our elliptical orbit results from the gravitational influences of the Sun and other planets, but principally by the Moon. Only one other planet, Pluto, has such a proportionately large moon with its major moon Charon, which together could almost be considered a double planet.
The 5 million km difference from perihelion to aphelion can be seen in a 3% change in the Sun’s apparent diameter as observed from Earth. Otago Museum’s Ian Griffin provided a clear example which featured as Astronomy Picture of the Day (APOD) for 9 January 2020.
Mars is visible form 6pm in the north-west at dusk before setting by 8.30pm. The Red Planet is not a bright object as its distance is currently 326 million km. When at its closest it can be 55 million km. See more at theskylive - Mars
Jupiter is an early object seen from 2am in the north-east before fading from view in the early dawn light.
Saturn can be seen pale and yellow from 10pm in the east and across the north during the night before being lost in the west by 7am.
Elongations and Conjunctions
For the inner planets Mercury and Venus, as viewed from Earth, there is an angular separation in our skies between them and the Sun. This is called their Elongation in the sense that their position ‘elongates’ away from the Sun as they orbit. At their Greatest Elongation they appear furthest from the sun as seen from Earth.
However, when they move behind the Sun their elongation is zero and that is called Superior Conjunction in that they are at greatest distance from Earth and appear ‘conjoined’’ in the sky with the Sun. When they pass in front of the Sun they are not visible but are at Inferior Conjunction and are at their least distance from us and again ‘joined’ with the Sun in our daytime sky.
Above is an ESO diagram depicting an inner planet (Mercury or Venus) at elongation and conjunction as if looking down on their orbits. These refer to a planet’s position as seen from Earth in relation to the Sun. Western Elongation occurs in our pre-dawn skies in the east, and Eastern Elongation in our evening skies in the west.
An outer planet (eg; Mars) is also shown but at opposition - on the opposite side of Earth to the Sun. At opposition it is a night time object, closest to us and at its brightest.
Find out more at:
EarthSky Astronomy Essentials - Elongation
EarthSky Astronomy Essentials - Conjunction
Meteors
This month we have no strong meteor showers but on the 27th-30th the Southern Delta Aquariids should be visible in the constellation of Aqaurius, the water bearer high in the north. The meteors will radiate from near Skat (Delta Auquarii), the fourth brightest star in the constellation. The best time to look is a few hours before dawn when perhaps twenty meteors per hour could be seen.
Find out more at:
EarthSky Delta Aquariid Meteor Shower
Stars and constellations
For evening viewing at local astronomical twilight when the sun is between 12 and 18 degrees below the western horizon. At this time, and depending on weather and the degree of light pollution, most visible celestial objects can be seen with the unaided eye.
In the west
Canis Major (and Sirius, brightest of the night time stars) is much lower this month in the west.
In the north
High in the north is Virgo and the star Spica, while Leo is in the north-west recognizable by its upside-down hook pattern of stars with the star Regulus.
In the east
Scorpius rises much higher in the south-east with the red-giant star Antares easily seen even from areas with city lights. Below and now fully revealed is the centaur-archer Sagittarius whose bow and arrow forms the famous asterism the ‘Tea Pot’.
In the south
The Southern Cross or Crux is high up directly south and, to the left, are the Pointers (Alpha and Beta Centauri which mark the front hooves of the other celestial centaur).
The billions of distant stars and numerous dark dust clouds of the Milky Way forms a broad band across the evening sky from east to west. From our southern perspective we enjoy a superb view of the galaxy quite different to most northern hemisphere locations.
Easy to see in the south-west are the intriguing irregularly shaped neighbouring galaxies, the Large and Small Clouds of Magellan, which are special features of our southern skies.
Visible are the bright stars Canopus in the south-west and Achernar closer to the horizon in the south.
International Space Station
ISS orbits every 90 minutes at an average distance of 400 km appearing like a bright star moving slowly across the night sky. Here are some of the brightest passes expected this month over Melbourne and Central Victoria:
Evening
- Wednesday 5th, 6.18pm – 6.24pm South-West to East
- Friday 7th, 6.17pm – 6.23pm West-South-West to North-East
Morning
- Wednesday 19th, 6.42am – 6.49am West-North-West to South-East
- Thursday 20th, 5.55am – 5:59am North-North-West to South-East
Heavens Above gives predictions for visible passes of space stations and major satellites, live sky views and 3D visualisations. Be sure to first enter your location under ‘Configuration’.
On this day
1st 1770, closest pass to Earth of any known comet: Comet Lexell at 2.2 million km (roughly 5.5 times the moon’s distance).
4th 1054, Chinese and other astronomers witness the supernova explosion that produced the Crab Nebula 6500 light years away.
4th 2005, Deep Impact probe (USA) crashes into Comet Tempel 1 to analyse its composition.
4th 1868, birth of Henrietta Swan Leavitt who established the luminosity-period relationship of Cepheid variable stars allowing Edwin Hubble to show ‘nebulae’ were other galaxies beyond our own.
5th 1687, Isaac Newton’s Mathematical Principles of Natural Philosophy is published laying the groundwork for much of modern science.
7th 1959, Venus’ diameter is determined and its atmosphere analysed by its occultation of the star Regulus in Leo.
8th 2011, the space shuttle Atlantis (USA) is launched on the final mission for the shuttle program.
10th 1962, first communications satellite Telstar (USA) is launched as an experiment in trans-Atlantic communication.
11th 1979, Skylab 1 (USA) is destroyed during re-entry over Western Australia and scatters debris over a wide area.
14th 1965, Mariner 4 (USA) makes the first controlled flyby of Mars and returned the first close-up images of the planet.
15th 1975, Apollo-Soyuz Test Project mission occurs. An American Apollo module with three astronauts (launched with the last Saturn rocket) and a Soviet Soyuz craft with two cosmonauts rendezvous and dock in Earth orbit.
16th 1746, birth of Giuseppe Piazzi, discoverer of the dwarf planet Ceres in the asteroid belt.
16th 1969, Apollo 11 (USA) launches to moon for the first lunar landing 5 days later.
16th 1994, Jupiter is struck by over twenty fragments of Comet Shoemaker-Levy 9 with Earth-based telescopes and the Hubble Space Telescope observing the unique event.
20th 1969, Apollo 11 (USA) Moon landing when Lunar Excursion Module ‘Eagle’ touched down in the Sea of Tranquility.
21st 1969, first humans set foot on another world. At 12:39pm AEST Neil Armstrong becomes the first person to step onto the Moon and is followed 20 minutes later by Buzz Aldrin, while the third Apollo 11 astronaut, Michael Collins, remains in the Command Service Module ‘Columbia’ in lunar orbit. Armstrong and Aldrin are on the moon for 21hrs 36min.
22nd 1784, first use of parallax and hence calculation of the distance to a star by Friedrich Bessel.
23rd 1995, Alan Hale and Thomas Bopp discover comet now named after them.
23rd 1928, Vera Rubin born – famous for analysing rotation rates of galaxies.
24th 1969, first lunar landing mission, Apollo 11, ends with Command Module splashdown in the Pacific Ocean.
30th 1971, first Lunar Rover used on the moon in Apollo 15 Mission.
29th 2005, dwarf planets, Eris and Makemake, are announced leading to Pluto becoming a dwarf planet as well.