Why Do We See More Shooting Stars After Midnight?
― The Pre-Dawn Sky Is the Earth's "Front Window"
News about meteor showers almost always says, "Best from after midnight until dawn." They aren't just trying to keep you up late. Once midnight passes, the place where you stand swings around to face the direction the Earth is speeding through space.
It's late October, the night of the Orionid meteor shower. You step onto the balcony at 9 p.m. and look up for 30 minutes. Not a single shooting star.
The next morning, a family member who got up around 4 a.m. says, "I saw three!" Same sky, same meteor shower. Only the time was different.
Were you just unlucky? Actually, no. The sky early in the evening and the sky before dawn face different directions.
There are two reasons for more meteors before dawn
The Earth travels around the Sun at about 30 kilometres per second. The pre-dawn sky faces that direction of travel, so we run head-on into bits of space dust.
Shower meteors appear to fly out from a single point among the stars. While that point is near the horizon, many of the meteors are hidden below the ground.
The first reason works on every ordinary night, shower or not. The second depends on the timing of each particular shower. Let's take them in turn.
Like a car in the rain, the front window gets hit the most
When you drive through rain, lots of drops hit the windscreen. The rear window gets almost none. That's because the car is moving into the raindrops.
Space is full of countless grain-sized specks. The Earth ploughs through them at about 30 kilometres per second. So where is the Earth's "front window"?
Look at Figure 1. The Earth also spins. As a result, the place that faces the direction of travel is the place where it is about 6 a.m. After midnight, the sky over your head gradually swings around toward the "front window".
The front window gets more than just a larger number of particles. The Earth's own speed adds to theirs, so they hit faster. The faster they hit, the brighter they glow, so even faint meteors become easy to see. Pre-dawn skies are said to show more than twice as many meteors as evening skies.
The meteors' "vent" climbs up the sky
A meteor shower happens when the Earth plunges into a stream of debris left along the path of a comet or similar body. The particles all arrive from the same direction on parallel paths, so they seem to fly outward from a single point in the sky. This point is called the "radiant".
It's the same as railway tracks seeming to meet at a distant point: an effect of perspective. The radiant rises in the east and sets in the west along with the stars.
Try changing the radiant's height with the slider in Figure 2. When the radiant is just above the horizon, nearly half the meteors head down into the ground and can't be seen. The higher the radiant climbs, the more the meteors spread across the whole sky, and the more you see.
The Orionid radiant lies near the constellation Orion. In Japan it rises in the east around 10 p.m. and is highest before dawn. So the first reason, the "front window", and the second, the "radiant's height", both favour the pre-dawn hours.
The radiant of the December Geminids is already up in the eastern sky by about 8 p.m. So they're said to be well worth watching from early in the evening. The trick is to check when each shower's radiant rises.
Summary
The pre-dawn sky is the "front window" of the Earth as it travels through space. We hit the particles head-on, so there are more of them, and they hit fast, so they glow brightly. In a meteor shower, the radiant where the meteors seem to come from also climbs higher as the night goes on.
You don't wait for shooting stars; you go out to meet them.
Before dawn, the Earth is diving face-first into a rain of debris.
You can read how a shooting star is only as big as a grain of sand in "How Big Is a Shooting Star?", and the history of people's long refusal to believe that stones fall from the sky in the "article on meteorites".
- On the night of a meteor shower, pick a safe spot with few street lights. Children must go with an adult and wear warm clothes.
- Around 9 p.m. and again around 4 a.m., look at a wide stretch of sky for 15 minutes each time and count the meteors you see. Don't count during the first few minutes while your eyes adjust to the dark.
- Reach out an arm and trace the path of a meteor backward. Trace several, and they should all seem to come from one point.
Bright moonlight or heavy cloud cuts the counts sharply. It's natural for numbers to vary, so try the comparison a few times.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from lower secondary school science to university courses
- Lower sec.Covered in lower secondary school science
- Upper sec.Covered in upper secondary "Basic Physics" and "Earth Science"
- Upper sec.+Advanced upper secondary material, or textbook sidebars
- UniversityUniversity-level specialist courses (astronomy, planetary science) not taught in upper secondary school
- ResearchTopics researchers are still investigating, not yet settled enough to be taught even at university
Lower sec.Terms: this phenomenon has names
- Meteor shower: when the Earth passes through a band of debris left in a comet's orbit, shooting stars become more frequent at about the same time every year.
- Radiant: the point in the sky from which a shower's meteors seem to fly out. Parallel particle paths look as if they meet at one point because of perspective.
- Apex: the point in the sky toward which the Earth is moving in its orbit. At about 6 a.m. local time, it passes high in the southern sky.
- Zenithal hourly rate (ZHR): the number of meteors one person would see in an hour under a clear, dark sky with the radiant straight overhead. It's a yardstick for comparing the "strength" of showers.
Lower sec.Upper sec.Checking with a formula: how many will you see in an hour?
The Orionids have a ZHR of about 20. Let's work out how the count changes with the radiant's height. We'll also calculate how much the impact speed differs between the front and rear windows.
| In symbols | N ＝ ZHR × sin h (impact speed v ＝ u ± V) |
| In words | Number seen per hour ＝ ZHR × sine of the radiant's height. Impact speed ＝ particle speed ± Earth's speed |
| Where it comes from | The same idea of "slanted incidence" as slanting rain, which delivers less water per unit area of flat ground. The speed part is relative velocity: add the speeds when facing each other, subtract when going the same way. |
| Meaning of symbols | N: number seen per hour, ZHR: zenithal hourly rate, h: radiant height (degrees) |
| Orionid ZHR | About 20 per hour |
| Sine of a 30° radiant height | 0.5 |
| Sine of a 70° radiant height (highest point near Tokyo) | About 0.94 |
| Earth's orbital speed V | About 30 km/s |
| Maximum speed at the Earth's position for staying in the Solar System, u | About 42 km/s |
| Radiant at 30° | 20 × 0.5 ＝ 10 per hour |
| Radiant at 70° (pre-dawn) | 20 × 0.94 ＝ 18.8 per hour |
| How many times the 30° count is the pre-dawn count? | 18.8 ÷ 10 ≒ 1.9 times |
| Maximum head-on speed at the front window | 42 ＋ 30 ＝ 72 km/s |
| Minimum catching-up speed at the rear window | 42 － 30 ＝ 12 km/s |
| Speed ratio | 72 ÷ 12 ＝ 6 times |
| Kinetic energy is proportional to speed squared, so | 6 × 6 ＝ 36 times |
Even for particles of the same size, those diving into the front window turn up to 36 times more energy into light and heat than those at the rear window. In reality the Earth's gravity speeds them up a little, so even the slowest meteors are said to move at about 11 km/s, and meteor speeds fall in a range of roughly 11 to 72 km/s. The Orionids, at about 66 km/s, are an especially fast shower.
Upper sec.Upper sec.+Adding speeds, and perspective
Upper sec.This is exactly the "relative velocity" taught in Basic Physics. The speed of a particle as seen from the Earth is the particle's velocity minus the Earth's velocity (vector subtraction, direction included). Kinetic energy is mass × speed² ÷ 2, so a difference in speed makes a large difference in brightness.
Upper sec.+The radiant is the same perspective effect as the "vanishing point" where parallel lines seem to meet in the distance. Its position is fixed with the shift in direction caused by the Earth's orbital motion taken into account. That's why the same shower appears in about the same constellation every year.
UniversityZHR corrections and parent bodies
To work out the zenithal hourly rate (ZHR) from observed counts, astronomers correct not only for radiant height but also for the faintest star visible to the naked eye that night (the limiting magnitude) and for the fraction of sky hidden by cloud. The Orionids and the May Eta Aquariids both have Halley's Comet as their parent body, the source of their debris. The Earth passes near Halley's orbit twice a year. The larger number of "sporadic meteors", which belong to no shower, before dawn is explained in terms of the direction of the Earth's apex.
📖 Derivations and further reading: Wikipedia, "Zenithal hourly rate" (Japanese) ／ Wikipedia, "Meteor shower" (Japanese)
ResearchWhat is still not clear
- Forecasting sudden outbursts Researchers are calculating the dust trails, the streaks of debris a parent comet leaves on each return, to forecast when meteors will suddenly surge in a particular year. More forecasts have come true, but predicting the actual numbers accurately is still said to be hard.
- Origins of sporadic meteors Meteors that belong to no shower are known to arrive in larger numbers from several directions in the sky. Which comet or asteroid each particle came from is still being worked out.
- Counting faint meteors Faint meteors the eye can't see are counted with radar and sensitive cameras. How to link the visual ZHR to machine counts is still debated.
In other words, this article too gives only "the explanation as far as we understand today". Meteor shower counts vary a lot from year to year, so take the numbers here as a rough guide.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Lower sec. | Science: Earth and space (rotation and revolution of the Earth) | The pre-dawn sky facing the Earth's direction of travel |
| Upper sec. | Basic Physics: relative velocity; Earth Science: small bodies of the Solar System | Adding and subtracting impact speeds |
| Upper sec.+ | Perspective and vanishing points; kinetic energy | Why the radiant looks like one point; differences in brightness |
| University | Astronomy and planetary science (meteor astronomy) | ZHR corrections, parent bodies, apex |
| Research | Dust trail calculations; origins of sporadic meteors | Forecasting outbursts; observing faint meteors |
| ― | Links to daily life | Choosing when to go out and watch a meteor shower |
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (list of meteor showers)
- National Astronomical Observatory of Japan (国立天文台), "Hoshizora Joho" (ほしぞら情報, Starry Sky Information) (notes on each meteor shower)
- International Meteor Organization, "Meteor Shower Calendar"
- Wikipedia, "Zenithal hourly rate" (天頂出現数)
- Takumi Nagasawa (長沢工), Ryusei to Ryuseigun (流星と流星群, Meteors and Meteor Showers), Chijin Shokan (地人書館)
Note: This article is a popular-science explanation. The numbers given are rough guides to help you understand the mechanism. When observing at night, stay on safe, level ground, and children should go with an adult.