This is the Saint Louis Science Center’s NIGHT SKY UPDATE for the week of Friday, April 23, 2021.
Information updated weekly or as needed.
Times given as local St. Louis time which is Central Daylight Time (CDT). For definitions of terminology used in the night sky update, click the highlighted text. If relying on times posted in Universal Time (UT), St. Louis is -5 hours when CDT.
Public Telescope Viewings
Star parties at the Saint Louis Science Center have temporarily been canceled due to recommendations from the CDC regarding COVID-19. All public telescope events are canceled until further notice. As conditions change, we will reevaluate and update this article once public observing events resume.
Observing Highlight of the Week
April 26, 2021 Full Moon
Credit: NASA, SVS, Ernie Wright
The Moon will reach its full phase this week on April 26 at 10:32 p.m. Usually, full moon is a deterrent for star gazers due to how bright it appears when near full moon. That said, full moon does offer us interesting topics and targets to explore. Although it may not affect us much in St. Louis, the Moon is connected to the ebb and flow of high and low tides. New and full moon phases are when we see the greatest effect on tides.
Visually, the brightness of full moon tends to mute the finer details on the lunar surface, due to the opposition effect. The opposition effect is why the Moon looks brighter at full moon, and it is related to shadows on the lunar surface. The shadows cast by the topography of the lunar surface grow longer or shorter with the changing solar altitude, much like how shadows cast on Earth have predictable behavior as the Sun climbs higher and then lower in the sky. Shadows will point in the opposite direction of the light source. When at full moon, we see the Moon opposite of where the Sun is. As a result, shadows being cast by lunar topography are pointed away from us, causing the Moon to appear brighter.
With these impaired viewing conditions, there are still interesting targets to track down on the lunar surface. Full moon is a great time to learn the lunar maria. Those large dark patches visible to the unaided eye are ancient lava flows that settled in low points on the lunar surface. Once such mare is called Imbrium. This large dark patch was the site of a huge impact that later filled with lava that erupted to the lunar surface.
Impact craters with rays are also great targets to look for at full moon. The rays that can be seen emanating from some craters are formed when material was ejected after an impact. The rays appear brighter than the surrounding area because is it a fresher surface material. A great example of this is Crater Tycho. The rays are also an indication that the crater is young, geologically speaking.
If you plan to observe the Moon near its full phase, it is recommended that you use a moon filter on your telescope. Moon filters are polarizing filters that can tone down the brightness of the Moon without losing details. This is recommended since the Moon can be quite uncomfortable to look at through a telescope near full moon.
The Sun and Moon
The Moon as seen from the International Space Station, on July 31, 2011.
Credit: NASA
Sunrise is at 6:13 a.m. on Friday, April 23 and sunset is at 7:46 p.m., providing us with about 13.5 hours of daylight. Even after sunset, the light from the Sun will dimly illuminate our sky for about 1 hour and 40 minutes. This period is called twilight, which ends around 9:24 p.m. this week. For those with a sundial, local noon occurs around 12:59 p.m. this week.
| Day | Sunrise | Sunset |
|---|---|---|
| 2021-04-23 | 6:13 a.m. | 7:46 p.m. |
| 2021-04-24 | 6:12 a.m. | 7:47 p.m. |
| 2021-04-25 | 6:11 a.m. | 7:48 p.m. |
| 2021-04-26 | 6:10 a.m. | 7:49 p.m. |
| 2021-04-27 | 6:08 a.m. | 7:50 p.m. |
| 2021-04-28 | 6:07 a.m. | 7:51 p.m. |
| 2021-04-29 | 6:06 a.m. | 7:52 p.m. |
| 2021-04-30 | 6:05 a.m. | 7:53 p.m. |
| 2021-05-01 | 6:03 a.m. | 7:53 p.m. |
Moon
Moonrise for Friday, April 23 occurs at 3:49 p.m., and moonset will occur at 12:12 a.m. the following morning. On Friday, April 23, the Moon will exhibit a waxing crescent phase with 83% of the lunar disk visible. Full moon occurs on April 26 at 10:32 p.m. The next lunar perigee occurs on April 27 at 10:23 a.m.
International Space Station (ISS) Observing
Visible passes of ISS from St. Louis for the week of April 23 occur during morning hours. The best passes this week occur on the mornings of April 26 and 29. Use the table below for information about these and other visible passes this week.
Catch ISS from St. Louis starting Friday, April 23
| Date | Starts | Max. altitude | Ends | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Time | Alt. | Az. | Time | Alt. | Az. | Time | Alt. | Az. | ||
| 24 Apr | -1.6 | 04:55:56 | 10 | S | 04:58:22 | 20 | SE | 05:00:55 | 10 | E |
| 26 Apr | -3.6 | 04:58:18 | 23 | SW | 05:00:12 | 65 | SE | 05:03:34 | 10 | ENE |
| 27 Apr | -2.7 | 04:12:52 | 35 | SE | 04:12:53 | 35 | SE | 04:16:01 | 10 | ENE |
| 28 Apr | -0.9 | 03:27:21 | 14 | E | 03:27:21 | 14 | E | 03:28:12 | 10 | E |
| 28 Apr | -2.9 | 05:00:19 | 20 | W | 05:02:15 | 41 | NNW | 05:05:29 | 10 | NE |
| 29 Apr | -3.8 | 04:14:44 | 68 | NW | 04:14:49 | 69 | NW | 04:18:11 | 10 | NE |
| 30 Apr | -1.7 | 03:29:05 | 27 | ENE | 03:29:05 | 27 | ENE | 03:30:47 | 10 | NE |
| 30Apr | -1.6 | 05:02:02 | 10 | WNW | 05:04:31 | 19 | NNW | 05:07:04 | 10 | NNE |
| 01 May | -2.2 | 04:16:20 | 25 | NW | 04:16:58 | 27 | NNW | 04:19:54 | 10 | NNE |
Magnitude (Mag): The Measure of brightness for a celestial object. The lower the value is, the brighter the object will be.
Altitude (Alt): The angle of a celestial object measured upwards from the observer’s horizon.
Azimuth (Az): The direction of a celestial object, measured clockwise from an observer’s location with north being 0°, east being 90°, south being 180° and west being 270°.
For information about ISS flyovers and other visible satellites, visit www.heavens-above.com
Detailed information regarding all unmanned exploration of our universe, missions past, present, and planned, can be found at Jet Propulsion Laboratories:
The Visible Planets
Looking West, at 8:30 pm, April 23, 2021
Credit: Stellarium, EG,
Looking Southeast, at 4:30 am, April 24, 2021
Credit: Stellarium, EG
This week, three naked-eye planets are visible. Mars is found high in the west after sunset. Jupiter and Saturn can be found in the southeast before sunrise.
Venus
Venus is starting another evening apparition. It is still a little too close to the Sun to see, but Venus should be easy to spot by the start of May shortly after sunset.
Mars
Currently, Mars appears as a 1.5-magnitude object that will be visible high in the west about 40 minutes after sunset. Mars sets by 12:20 a.m.
Jupiter
Jupiter is visible in the southeast before sunrise. Jupiter rises at 3:35 a.m. and will be easy to see in the southeast by 4:30 a.m.
Saturn
Saturn has returned to our morning sky. Saturn rises at 2:54 a.m. and will be easy to spot by 4:00 a.m. looking southeast.
James S. McDonnell Planetarium
Night Sky Update: April 23-May 1, 2021
