Night sky, December 2023: What you can see tonight [maps]

The night sky tonight and on any clear night offers an ever-changing display of fascinating objects you can see, from stars and constellations to bright planets, the moon, and sometimes special events like meteor showers.

Observing the night sky can be done with no special equipment, although a sky map can be very useful, and a good telescope or binoculars will enhance some experiences and bring some otherwise invisible objects into view. You can also use astronomy accessories to make your observing easier, and use our Satellite Tracker page powered by N2YO.comto find out when and how to see the International Space Station and other satellites. We also have a helpful guide on how you can see and track a Starlink satellite train. 

You can also capture the night sky by using any of the best cameras for astrophotography, along with a selection of the best lenses for astrophotography. 

Read on to find out what's up in the night sky tonight (planets visible now, moon phases, observing highlights this month) plus other resources (skywatching terms, night sky observing tips and further reading)

Related: The brightest planets in December's night sky: How to see them (and when)

Monthly skywatching information is provided to Space.com by Chris Vaughan of Starry Night Education, the leader in space science curriculum solutions. Follow Starry Night on Twitter @StarryNightEdu and Chris at @Astrogeoguy

Editor's note: If you have an amazing skywatching photo and would like to share them with Space.com's readers, send your photo(s), comments, and your name and location to spacephotos@space.com.

Calendar of observing highlights

Friday, Dec. 1: Bright moon joins Gemini's Twins (before sunrise) 

Early risers on Friday morning, December 1 can enjoy the sight of the bright star Pollux sparkling several finger-widths to the lower right (or 4 degrees to the celestial northwest) of the waning gibbous moon in the western sky. Pollux' "twin", the double star Castor, will be shining off to their right, forming a bent line with them. 

If you have difficulty seeing the stars, position the bright moon beyond the left edge of your binoculars' field of view. You can watch the moon and the two stars cross the night sky from east to west starting late on Thursday evening. Once the sun rises, the stars will disappear, leaving the pale moon to haunt the morning daytime sky like an echo of the night.  

Monday, Dec. 4: Mercury at greatest eastern elongation (after sunset) 

On Monday, Dec. 4, Mercury (orbit shown in red) will reach its widest separation of 21 degrees east of the sun and maximum visibility for its current evening apparition. With Mercury positioned well below the tilted evening ecliptic (green line) in the southwestern sky, this appearance of the planet will be so-so for Northern Hemisphere observers, but an excellent one for those in the tropics and the Southern Hemisphere. 

The optimal viewing times at mid-northern latitudes will start around 5 p.m. local time. Viewed in a telescope (inset) tonight the planet will exhibit a waning gibbous phase. 

Tuesday, Dec. 5: Third Quarter Moon (at 12:29 a.m. EST) 

The moon will complete three-quarters of its orbit around Earth, measured from the previous new moon, on Tuesday, December 5 at 12:49 a.m. EST (0549 GMT). 

At the third (or last) quarter phase the moon is half-illuminated, on its western, sunward side. It will rise around midnight local time, and then remain visible until it sets in the western daytime sky in early afternoon. 

Third quarter moons are positioned ahead of the Earth in our trip around the sun. About three and a half hours later, Earth will occupy that same location in space. The week of dark, moonless evening skies that follow this phase are the best ones for observing fainter deep sky targets. 

Wednesday, Dec. 6: Neptune reverses course (evening) 

On Wednesday, Dec. 6, the distant, blue planet Neptune will complete a retrograde loop that has been carrying it slowly westward through the stars on the border between Aquarius and Pisces since July 1. After ceasing its motion tonight, Neptune will ramp up to its regular eastward motion over the coming days.

On moonless evenings in December the magnitude 7.9 planet can be observed in good binoculars and backyard telescopes in the sky between Jupiter and Saturn. Place, Lambda and Kappa Piscium, the lowest two stars of the circlet of Pisces, just outside the top of your binoculars' field of view (orange circle) and look for blue Neptune near the bottom of the field. Or, search less than a binoculars' field width to the right (or celestial west) of the box formed by the four medium-bright stars 27, 29, 30, and 33 Piscium. 

Thursday, Dec. 7: Stellar halo around Mirfak (all night) 

On mid-December evenings the constellation of Perseus is climbing the northeastern sky. Just for 2023, bright Jupiter will also be gleaming to Perseus' right. The outer rim of our Milky Way galaxy runs through Perseus' stars, filling its territory with rich star clusters. The largest of those surrounds his brightest star, Mirfak, or Alpha Persei. That elderly yellow supergiant star has evolved out of its blue phase and is now fusing helium into carbon and oxygen in its core. 

Melotte 20, also known as the Alpha Persei Moving Group and the Perseus OB3 Association, is a collection of 100 or so young, massive, hot B- and A-class stars sprinkled over several finger widths (or 3 degrees) of the sky around Mirfak. The cluster can be seen with unaided eyes, but it's especially dazzling in binoculars (orange circle). Its stars are approximately 600 light years from the sun and are moving as a group — Mirfak along with them.

Saturday, Dec. 9: Crescent moon visits Venus (before sunrise) 

When the brilliant planet Venus rises in the east around 3:30 a.m. local time on Saturday, Dec. 9, it will be accompanied by the pretty crescent of the old moon. The duo will share the field of view with binoculars and will make a terrific photo opportunity when composed with some interesting foreground scenery. They'll remain visible climbing the southeastern sky until the sky brightens at sunrise. 

Monday, Dec. 11: Watch Algol fade (starting at 7:34 pm EST) 

Algol, also designated Beta Persei, is among the most easily observed variable stars for skywatchers. During a ten-hour period that repeats every 2 days, 20 hours, and 49 minutes, Algol's visual brightness dims and re-brightens noticeably when a companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star, reducing the total light output we receive. 

Algol normally shines at magnitude 2.1, similar to the nearby star Almach (aka Gamma Andromedae). But when fully dimmed, Algol's brightness of magnitude 3.4 is almost identical to Rho Persei (or Gorgonea Tertia or ρ Per), the star that sits just two finger widths to Algol's lower right (or 2.25 degrees to the celestial south). Around 7:34 p.m. EST on Monday evening, Dec. 11, Algol will commence one of its drops in brightness. At that time it will be shining relatively high in the eastern sky between the bright star Mirfak and much brighter Jupiter. When Algol reaches its minimum intensity five hours later, at 12:34 a.m. EST on Tuesday morning, it will be located high in the west-northwestern sky. Algol's variations are best seen with unaided eyes and binoculars, which allow you to see its comparison stars at the same time.

Tuesday, Dec. 12: New Moon (at 6:32 p.m. EST (2332 GMT) 

The moon will reach its new phase on Tuesday, Dec. 12 at 6:32 p.m. EST, 3:32 p.m. PST, or 23:32 GMT. At that time our natural satellite will be located in Ophiuchus, 4.9 degrees south of the sun. While new, the moon is traveling between Earth and the sun. Since sunlight can only reach the far side of the moon, and the moon is in the same region of the sky as the sun, the moon becomes unobservable from anywhere on Earth for about a day (except during a solar eclipse). On the evenings following the new moon phase, Earth's planetary partner will return to shine in the western sky after sunset.  

Wednesday, Dec. 13: Geminids Meteor Shower Peak (overnight) 

The Geminids, one of the most spectacular meteor showers of the year, is active from Nov. 19 to Dec. 24 annually. The number of Geminids meteors will be gradually ramping up until a peak on Thursday, Dec. 14. Then the numbers will decline rapidly on the following nights. 

Geminid meteors are often bright, intensely colored, and slower-moving than average because they are produced by sand-sized grains dropped by the asteroid designated 3200 Phaethon. In the Americas, expect to see a good number of Geminids meteors beginning after dark on Wednesday evening, Dec. 13. Upwards of 120 meteors per hour are possible around 2 a.m. local time on Thursday — the time when the sky overhead will be pointing toward the densest part of the debris field. 

Geminids will continue to appear until twilight begins on Thursday morning. Thursday night should be good, too. True Geminids will appear to streak away from a position above the bright stars Castor and Pollux, but the meteors can appear anywhere in the sky, so just keep looking up and around. In 2023, there will be no moonlight to obscure the fainter meteors. 

Thursday, Dec. 14: Mercury and the young moon (after sunset) 

Look just above the southwestern horizon after sunset on Thursday, Dec. 14 to see the planet Mercury positioned a fist's diameter to the right (or 10.5 degrees to the celestial west-northwest) of the young crescent moon. 

Binoculars will aid in your search, but wait until the sun has completely set before turning any optical aids towards the western horizon. Mercury will set at about 5:30 p.m. local time — the moon about 25 minutes later. Skywatchers around the globe at or below the latitudes of the southern USA will see the pair more easily.

 Friday, Dec. 15: Earthshine (after sunset) 

On the evenings surrounding Friday, Dec. 15, the young crescent moon will shine low in the southwestern sky after sunset. Watch for Earthshine, also known as the Ashen Glow and "the old moon in the new moon's arms". That's sunlight reflected off Earth and back onto the moon, slightly brightening the dark portion of the moon's Earth-facing hemisphere. 

The phenomenon appears for several days after each new moon but is strongest in springtime at mid-northern latitudes when the moon is positioned directly above the sun when it sets.

Saturday, Dec. 16: The Hyades Cluster (all night) 

Located only about 150 light-years away from the sun, Taurus, the Bull's triangular face is actually one of the nearest open star clusters to our solar system. 

Its stars are commonly called The Hyades, named for the five daughters of Atlas in Greek mythology. It also has the designations Melotte 25 and Caldwell 41. The cluster contains several hundred stars, with a half-dozen or so readily seen under moonless suburban skies, many as close-together pairs. 

It's a superb target to view in binoculars (orange circle). The five brightest members, all naked-eye stars, are within a few light years of one another. The cluster's stars likely formed together about 625 million years ago. The bright orange star Aldebaran, at the lower (or southeastern) vertex of the Hyades triangle, is actually not part of the cluster. It is less than half as far away! In mid-December, the Hyades climbs the eastern sky in the early evening and reaches its highest point due south by 11 p.m. local time.

Sunday, Dec. 17: Crescent moon near Saturn (evening) 

The waxing moon's monthly cruise past the evening planets will commence on Sunday, Dec. 17, when the pretty, 29%-illuminated crescent moon will shine only a few finger widths below (or 3 degrees to the celestial south of) Saturn. They'll be cozy enough to share the field of view in binoculars (orange circle) until they set in the west before 10 p.m. local time. By then, the moon's easterly motion and the diurnal rotation of the sky will shift Saturn to the moon's right. 

Sunday, Dec. 17: Watch Algol brighten (starting at 6:17 pm EST) 

Another fine opportunity to watch the star Algol vary in brightness will arrive on Sunday evening, Dec. 17 in the Americas. At 6:17 p.m. EST, the star will be positioned about halfway up the eastern sky, shining at its minimum brightness of magnitude 3.4, which is almost identical to Rho Persei (or Gorgonea Tertia or ρ Per), the star that sits just two finger widths to Algol's lower right (to the celestial south). 

Five hours later at 11:17 p.m. EST, Algol will be high in the western sky and shining at its full intensity of magnitude 2.1, similar to the star Almach (aka Gamma Andromedae), which will be located a generous fist's diameter below Algol. To find other opportunities to watch Algol fade, or to re-brighten, search the web for "minima of Algol" times.  

Monday, Dec. 18: The Double Cluster (all night)  

The northeastern sky on December evenings hosts the bright constellations of Perseus and W-shaped Cassiopeia, with the very bright star Capella positioned below them. The sky between Perseus and Cassiopeia hosts the Double Cluster, a pair of bright open star clusters that together cover a finger's width of the sky. 

They make a spectacular sight in binoculars (orange circle) or a telescope at low magnification. The upper left (more westerly) cluster, designated NGC 869, is dense and contains more than 200 white and blue-white stars. The lower right (easterly) cluster NGC 884 is looser and includes a handful of 8th-magnitude golden stars. The clusters, which formed in the same part of the Perseus Arm of the Milky Way galaxy, are about 7,300 light-years away from us. The clusters would be even brighter if they weren't being dimmed by opaque dust in the galactic plane.

Tuesday, Dec. 19: First Quarter Moon (at 1:39 p.m. EST) 

The moon will complete the first quarter of its 29.53-day journey around Earth on Tuesday, Dec. 19 at 1:39 p.m. EST (10:39 a.m. PST, or 18:39 GMT). At first quarter, the moon's 90-degree angle from the sun causes us to see it half-illuminated on its eastern side. 

First quarter moons always rise around mid-day and set around midnight, so they are also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best ones for seeing the lunar terrain when it is dramatically lit by low-angled sunlight, especially along the terminator, the pole-to-pole boundary that separates the moon's lit and dark hemispheres. 

Thursday, December 21: Minor planet Vesta at opposition (all night) 

On Thursday, Dec. 21, the Earth's orbital motion will carry us between the minor planet Vesta and the sun. Because it will be opposite the sun in the sky, Vesta will be visible all night long, and shine at its brightest for the year (magnitude 6.2) — within reach of binoculars (orange circle) and small telescopes. 

Tonight look for the asteroid near the raised club of Orion, below the midpoint between Nu Geminorum, the star that marks Castor's lower foot and Zeta Tauri, the lower horn-tip star of Taurus. Vesta's trajectory over the ensuing couple of weeks will carry it toward Zeta Tauri (red dotted line).  

Thursday, Dec. 21: Northern Winter Solstice (at 10:27 P.M. EST) 

Winter in the Northern Hemisphere will officially commence in the Americas on Thursday, December 21 at 10:27 p.m. EST and 7:27 p.m. PST, which converts to 03:27 GMT on Friday, Dec. 22. At the solstice, the sun will attain its southernmost declination for the year, resulting in the lowest elevation in the sky of the noonday sun, the least amount of solar insolation, and the shortest amount of daylight of the year. 

Conversely, the Southern Hemisphere will see the highest sun and maximum daylight hours for the year. After the December solstice, the amount of daylight time will begin to increase for the Northern Hemisphere.

Thursday, Dec. 21: Gibbous moon jumps past Jupiter (all night) 

Even before the sky fully darkens on Thursday, Dec. 21, the bright, waxing gibbous moon will catch your eye in the southeastern sky. The dazzling planet Jupiter will be shining to the moon's lower left. 

As they cross the sky together overnight, the moon's easterly orbital motion will carry it closer to Jupiter. Meanwhile, the diurnal rotation of the sky will swing Jupiter above the moon before they set in the west around 2:45 a.m. local time on Friday morning. 

On Friday evening, the show will repeat with the moon now to Jupiter's left (celestial east) and the planet below the moon when they set in the wee hours.

Friday, Dec. 22: Ursids meteor shower peak (overnight) 

The annual Ursids meteor shower, produced by debris dropped by periodic comet 8P/Tuttle, runs from Dec. 13 to 24. The short-duration shower will peak with 5 to 10 meteors per hour while Earth is traversing the densest part of the debris field on Friday evening, Dec. 22 in the Americas, but the best time to watch for Ursids will be the hours after midnight on Saturday morning. 

True Ursids will appear to travel away from a position in the sky near Polaris in the Little Dipper (Ursa Minor), but the meteors can appear anywhere in the sky. The shower's peak night will host a bright moon that will hide the fainter meteors until it sets around 4 a.m. local time. 

Saturday, Dec. 23: Two shadows cross Jupiter  

From time to time, observers with good-quality telescopes can watch the small, round, black shadows of the Galilean moons traverse Jupiter's disk. 

On Saturday evening, Dec. 23, sky-watchers located in the United Kingdom, Europe, and Africa can watch two shadows crossing the southern hemisphere of Jupiter at the same time for about 25 minutes. At 8:25 p.m. Central European Time, the small shadow of Europa will join the much larger shadow of Ganymede, which began its own crossing of the planet's south polar zone 70 minutes earlier. Ganymede's shadow will leave Jupiter at 8:49 p.m. CET, leaving Europa's shadow to continue on alone until 10:42 p.m. CET. Watch for Europa itself to move off of Jupiter's disk by 8:35 p.m. CET.  

Sunday, Dec. 24: The moon crosses the Winter Football (all night) 

The Winter Football, also known as the Winter Hexagon and Winter Circle, is an asterism composed of the brightest stars in the constellations of Canis Major, Orion, Taurus, Auriga, Gemini, and Canis Minor — specifically Sirius, Rigel, Aldebaran, Capella, Castor & Pollux, and Procyon. 

All of those stars will have cleared the horizon in the southeastern sky by 7 p.m. local time. When it stands upright in the south towards midnight, the great pattern will encompass an area of the sky 45 degrees wide and 66 degrees high. The shape is visible during the evening from mid-November to spring every year. 

On Sunday, Dec. 24, the bright, nearly full moon will shine on the right-hand (western) edge of the football, between Capella and Aldebaran. The moon will journey through the football until Wednesday night (green arc). When the moon isn't so bright, the Milky Way can be traced vertically through the asterism.

Monday, Dec. 25: A Christmas star (late evening) 

Sirius, the brightest star in Canis Major and in the entire night sky, will appear above the southeastern horizon by 7:30 p.m. local time in late December. It's hard to miss Sirius once it clears the trees and rooftops. The star will climb to its highest point, in the lower part of the southern sky, shortly after midnight. 

If you are walking through your darkened house in the middle of the night, Sirius might catch your eye out a window because it never climbs very high. Sirius is a hot, blue-white, A-class star located only 8.6 light-years from the sun. Its extreme brightness and its low position in the sky combine to produce spectacular flashes of color as it twinkles. A very large telescope may allow you to see Sirius B, a faint white dwarf companion star located just 10 arc-seconds east of Sirius.

Tuesday, Dec. 26: Full Oak Moon  

The December full moon will occur at 7:33 p.m. EST (4:33 p.m. PST) on Tuesday, Dec. 26 in the Americas. That converts to 00:33 GMT on Wednesday, Dec. 27. 

Traditionally known as the Oak Moon, Cold Moon, and Long Nights Moon, it always shines in or near the stars of Taurus and Gemini. The Ojibwe of the Great Lakes region call the December full moon Manidoo Giizisoons, the "Little Spirit Moon". For them it is a time of purification and of healing of all Creation. Since it's opposite the sun on this day of the lunar month, the moon is fully illuminated and rises at sunset and sets at sunrise. Full moons during the winter months reach as high in the sky at midnight as the summer noonday sun, and cast similar shadows. 

Thursday, Dec. 28: The Moon's western region (all night) 

The left-hand (western) half of the moon's Earth-facing hemisphere is dominated by the large and irregular Oceanus Procellarum "the Ocean of Storms" and round Mare Imbrium "the Sea of Rains", which adjoins Procellarum to the lunar northeast. Both are ancient basins excavated by huge impactors and later flooded with dark, iron-rich basalts that upwelled from the moon's interior.

The lunar maria are far less cratered than the bright highlands, but they are not featureless. Binoculars and telescope views reveal that the basalts vary in color and darkness due to their chemistry. When the lunar terminator is near a mare, slanted sunlight casts shadows from curved wrinkle ridges that ring each mare's interior.

Saturday, Dec. 30: Double shadows on Jupiter (5:17 to 6:53 p.m. EST)  

Another pair of the small, round, black shadows cast by the Galilean moons will traverse Jupiter's disk on Saturday evening, December 30. Skywatchers with good-quality telescopes in eastern USA and Canada, eastern South America, and the United Kingdom, Europe, and Africa can watch the shadows of Europa and Ganymede cross the southern hemisphere of Jupiter at the same time for almost two hours. 

At 5:17 p.m. EST, the large shadow of Ganymede will join the smaller shadow of Europa, which began its own crossing of the planet 15 minutes earlier. Ganymede's shadow will leave Jupiter at 6:53 p.m. EST, leaving Europa's shadow to continue on alone until 7:18 p.m. EST.

Sunday, Dec. 31: Jupiter completes a retrograde Loop (all night) 

On Sunday, Dec. 31, the very bright planet Jupiter will temporarily cease its motion (dotted line) through the distant stars of southern Aries — marking the end of a westward retrograde loop that it began in early September. 

After tonight the planet will ramp up to its regular eastward motion, which will carry it into Taurus for 2024. Retrograde loops occur when Earth, on a faster orbit closer to the sun, passes more distant solar system objects "on the inside track", making them appear to move backward across the stars for a period of time. Jupiter's loop spanned about a fist diameter or 10 degrees along the ecliptic.

Visible planets in December

Mercury

Mercury will begin December in the western sky after sunset. It will be in the latter stages of a so-so evening apparition for Northern Hemisphere observers, but a good showing for southerly viewers. On Dec. 4, Mercury will reach its widest separation of 21 degrees east of the sun and maximum visibility. 

The speedy planet will become unobservable by mid-December, pass the sun at inferior conjunction on Dec. 22, and then commence a fine apparition with Venus in the southeastern pre-dawn sky at month's end.

Venus

The brilliant planet Venus will continue its domination of the southeastern pre-dawn sky during December, but it will be descending daily as it treks sunward. At the beginning of the month, the magnitude -4.2 "morning star" will rise in central Virgo at around 3:45 a.m. local time. 

In a telescope, the planet will display a 67%-illuminated, football shape and an apparent disk diameter of 17 arc-seconds. With each passing day, Venus will slightly increase in illuminated phase, shrink in size, and diminish in brightness. It will cross through Libra from Dec. 8 to 31, ending the month rising just before 5 a.m. local time near the claw stars of Scorpius and showing a 78%-illuminated, 14 arc-seconds-side disk — with much fainter Mercury positioned 20 degrees to Venus' lower left (or celestial east). The old crescent moon will shine close to Venus on Dec. 9. 

Mars

Recently past its November solar conjunction, Mars will spend December hidden by morning twilight in the southeastern pre-dawn sky while it increases its angle from the sun. The magnitude 1.4 red planet may be spotted with difficulty by the end of December. Mars will pass several degrees to the lower right (or celestial south) of brighter Mercury on the mornings surrounding Dec. 28.  

Jupiter

Very bright, magnitude -2.7 Jupiter will be well-placed for observing from dusk through most of the night during December. As the days advance, and its distance to Earth increases, Jupiter will diminish in brightness and shrink in apparent size. 

Binoculars will reveal Jupiter's four large Galilean moons flanking the planet on any night, and views of Jupiter in a backyard telescope will show its equatorial zones and belts. Better quality optics will reveal the Great Red Spot every 2nd or 3rd night. Jupiter's Galilean satellites will frequently eclipse and occult one another, and the round, black shadows they cast upon the planet can be seen crossing Jupiter's disk — in pairs on Dec. 23 and 30. 

On Dec. 31, Jupiter will cease its westward motion through the stars of southwestern Aries — marking the end of a retrograde loop that it began in early September. The waxing gibbous moon will hop past Jupiter on December 21-22.

Saturn

Despite its shift west and sunward, the early sunsets of December will allow Saturn to be observed by mid-northern latitude observers during the evening all month long. It will look clearest in a telescope right after dusk, while it is still reasonably high in the southwestern sky. 

The yellowish planet will be traveling eastward through the stars of central Aquarius, decreasing in brightness and size as Earth moves farther away from the planet. 

Saturn's rings, which will be tilting more edge-on to Earth every year until the spring of 2025, are noticeably less wide this year. Quality optics and good seeing conditions may allow you to see the dark, narrow Cassini Division dividing the rings. Watch for a handful of Saturn's moons arrayed as tiny dots around the planet. Its outermost large moon Iapetus will vary in brightness when its light and dark hemispheres are turned toward Earth around Dec. 1 and 31, respectively. The waxing crescent moon will shine several degrees below (or south of) Saturn on Dec. 17.  

Uranus

Only weeks past opposition, Uranus will be an all-night target during December — especially in late evening, when it will be highest in the sky and appear most clearly in a backyard telescope or binoculars. The magnitude 5.65 planet has been moving retrograde westward in Aries every night, following Jupiter across the sky in a location about midway between Jupiter and the Pleiades cluster. 

Binoculars users can use the 4th-magnitude stars Zeta and Delta Arietis shining several degrees to Uranus' upper left (or celestial NNE) to aid in their search. Uranus will show a 3.8 arc-seconds-wide, blue-green disk in telescopes. On Dec. 22-23 the very bright, nearly-full moon will shine nearby.

Neptune

The early December sunsets are also helping to keep distant Neptune available for viewing in the southwestern sky during the evening. The magnitude 7.9 planet has been trailing Saturn across the sky this year. It will complete a retrograde loop on Dec. 6-7 and then resume its regular eastward motion near the border of Aquarius, just to the south of the circle forming the western fish of Pisces. 

In a telescope, Neptune's tiny blue disk will span 2.3 arc-seconds. Larger telescopes can also reveal its large moon Triton. The first quarter moon will shine to the upper left (or celestial east) of Neptune on Dec. 19.

Skywatching terms

Gibbous: Used to describe a planet or moon that is more than 50% illuminated.

Asterism: A noteworthy or striking pattern of stars within a larger constellation.

Degrees (measuring the sky): The sky is 360 degrees all the way around, which means roughly 180 degrees from horizon to horizon. It's easy to measure distances between objects: Your fist on an outstretched arm covers about 10 degrees of sky, while a finger covers about one degree.

Visual Magnitude: This is the astronomer's scale for measuring the brightness of objects in the sky. The dimmest object visible in the night sky under perfectly dark conditions is about magnitude 6.5. Brighter stars are magnitude 2 or 1. The brightest objects get negative numbers. Venus can be as bright as magnitude minus 4.9. The full moon is minus 12.7 and the sun is minus 26.8.

Terminator: The boundary on the moon between sunlight and shadow.

Zenith: The point in the sky directly overhead.

Night sky observing tips

 Adjust to the dark: If you wish to observe fainter objects, such as meteors, dim stars, nebulas, and galaxies, give your eyes at least 15 minutes to adjust to the darkness. Avoid looking at your phone's bright screen by keeping it tucked away. If you must use it, set the brightness to minimum — or cover it with clingy red film. 

Light Pollution: Even from a big city, one can see the moon, a handful of bright stars, and the brightest planets - if they are above the horizon. But to fully enjoy the heavens — especially a meteor shower, the fainter constellations, or to see the amazing swath across the sky that is the disk of our home galaxy, the Milky Way — rural areas are best for night sky viewing. If you're stuck in a city or suburban area, use a tree or dark building to block ambient light (or moonlight) and help reveal fainter sky objects. If you're in the suburbs, simply turning off outdoor lights can help.

Prepare for skywatching: If you plan to be outside for more than a few minutes, and it's not a warm summer evening, dress more warmly than you think is necessary. An hour of winter observing can chill you to the bone. For meteor showers, a blanket or lounge chair will prove to be much more comfortable than standing, or sitting in a chair and craning your neck to see overhead.

Daytime skywatching: On the days surrounding first quarter, the moon is visible in the afternoon daytime sky. At last quarter, the moon rises before sunrise and lingers into the morning daytime sky. When Venus is at a significant angle away from the sun it can often be spotted during the day as a brilliant point of light - but you'll need to consult an astronomy app to know when and where to look for it. When large sunspots develop on the sun, they can be seen without a telescope — as long as you use proper solar filters, such as eclipse glasses. Permanent eye damage can occur if you look at the sun for any length of time without protective eyewear.