# Article usage: "to the speed of 8 km…" vs "to a speed of 8 km…"

I have to take the TOFEL soon, and encountered a sentence from a textbook which made me think. In the case below, instead of using a as a preposition, is using the wrong?

Quote: To deliver a satellite into orbit, a rocket has to accelerate the satellite to the (originally it was “a”) speed of 8 kilometers a second more than 30 times than the speed of a jet aircraft.

I am wondering the text is using the preposition "the" when it is talking about the speed of a jet, but it is using the preposition a in succeeding case.

In this case, the succeeding paragraph is talking about the speed in general, so am I wrong?

I encountered another same thing. The phrase of the text is,

Light travels through a vacuum at a speed of nearly 3*108 m/sec

So a might be a winner with a slight edge.

• A and the are determiners, not prepositions.
– user230
Jun 12, 2018 at 12:07

I'm struggling to analyse why one of these is more acceptable than the other, but imo, it does have to be
"a speed of 8km/s"

Yes, it's discussing a speed very specific to its purpose of leaving the gravity well, achieving 'escape velocity', which would make you think it ought to be
"the speed of 8km/s"
but I think 8km/s is just a bit 'vague' - it's not "the speed of sound" or "the speed of light", it's just any old speed really. It sounds no more 'special' than 7km/s or 9km/s.

A car can't travel at the speed of sound, but it can travel at a speed of 50km/h or a speed of 100km/h, or anywhere in between. One of a number of speeds it could be driven at, none more special than any other.
You could break the speed limit by travelling faster than allowed; that would be specific to that particular stretch of road.

I don't think in this case that "the speed of 8km/s" is 'wrong', it just feels 'less good' in this context.

• So, if I take your theory, if a car could travel to --the-- speed of 1,000km a second, which is very "special", would it be "the car travels to the speed of 1,000km a second?"
– user17814
May 27, 2018 at 9:01
• Not to my mind. Even taking a more realistic, but still 'special' number - this car can reach a speed of 300km/h... it's still not 'special' enough to be 'the'. 'The speed of sound' is special enough to have a name... even if you don't actually know what the speed is, you know the name. May 27, 2018 at 9:06
• I don't understand...I am afraid to say, it's kind of your personal theory that has no specific source..
– user17814
May 27, 2018 at 9:29
• Well, the 'specific source' is 60 years of speaking it as a native. If you're not happy with it as an answer, feel free to wait for an alternative to be posted. May 27, 2018 at 9:33

This is an interesting question. I agree with Tetsujin's answer that it should be a speed, and his explanation rings true for speed. But the rule or practice must be more complicated than that, because it doesn't hold in all cases. I couldn't find an official rule, but there seems to be a pattern. I'll toss this out to see if anyone can find exceptions that would prove it wrong.

When you talk about measures of things, the is used with measures that are done in, or are treated as, whole numbers. A few examples:

At the count of 3, you'd better be in your room!
My child knew the names of all the US presidents at the age of 7.

Counting is done in whole numbers. We talk about people's age in whole years; you turn 7 and then you're 7 until you turn 8.

When you talk about measures that are done on, or are treated as, a continuous scale, a is used. A few examples:

To deliver a satellite into orbit, a rocket has to accelerate the satellite to a speed of 8 kilometers a second.
This relic has an age of 3500 years. (Historical chronology is treated as a continuous number.)
The chute is set to open at an altitude of 2,000 feet.
He broke the record, lifting a weight of 1,000 pounds.
The glass shattered at a frequency of 1243 Hz.
The moon is at a distance of 252,088 miles.
Lesions can be cryosurgically removed at a temperature of -42 degrees F.
Sound at a loudness of 120 dB is painful.

All of these examples are stated as very precise whole numbers, but they are on a continuous scale. We treat them as if they have been rounded to a whole number for simplicity.

I personally think it is incorrect to say "the speed of" in this context.

I'm sure you understand that "the" is the definite article which would denote that there is only one of something, or you are referring to a specific example of something.

a rocket has to accelerate the satellite to the speed of 8 kilometers a second

This just sounds wrong. The rocket won't be the only thing traveling at 8km per second, so how can that speed be the definite article?

I can see why some people might think it might be okay. Let's look at a sentence that uses "to the" in a similar way:

Several songs are sung to the tune of 'Twinkle Twinkle Little Star'.

This means that more than one song can be sung to "the tune" (like lots of rockets accelerating to the same speed) but the sentence also makes it very clear that "the tune" belongs to one particular named song.

The only time I can think of "the speed" being used correctly is the speed of light (or sound, but that varies depending on air temperature so lets keep it simple). But by saying something is traveling (or accelerating to) the speed of light you are actually comparing it to something specific which is why the definite article is required. The speed 299,792 kilometers per second is the same speed as light travels.

So back to your example, and saying:

the speed of 8 kilometers a second

..is in effect saying that the speed of 8km/s is the speed that belongs to 8km/s. That makes no real sense.

In the latter part of your example, where it goes on to compare the speed to "the speed of a jet aircraft" - this is a specific, defined speed, and is also a comparison. There may be many jet aircraft, but they all have the same maximum speed. So the speed of a jet aircraft is a comparison to the specific speed shared by all jet aircraft.

• But, but then according to your explanation about a rocket, here we can see at the last, more than 30 times than the speed of a jet aircraft.. There are thousands of jet aircrafts then...
– user17814
May 26, 2018 at 8:59
• @KentaroTomono Not sure if I have just been complimented or patronised. But "the speed of a jet aircraft" is a specific, defined speed and is a comparison. There may be many jet aircraft, but they all have the same maximum speed. So the speed of a jet aircraft is a comparison to the specific speed shared by all jet aircraft. May 26, 2018 at 9:27
• I was not "patronizing" you, sorry. What I knew for sure was that to native speakers a would be more suitable by their first intuition.
– user17814
May 26, 2018 at 9:37
• I downvoted because (a) I disagree that using “the” in this context would be “incorrect,” (b) I disagree with the reasoning (that the definite article would denote that there is only one of something, or the you are referring to a specific example of something). Such oversimplifications perpetuate learner confusion; the word “the” has plenty of other uses. Moreover, instances of using the with rates of speed are readily found in textbooks, e.g.: A BS has a 900 MHz transmitter and a vehicle is moving at the speed of 50 mph...
– J.R.
May 26, 2018 at 11:28
• @J.R. Interestingly in your example it is "a" vehicle moving at "the" speed of.... try saying that with "the" in both places and tell me that doesn't sound wrong. May 26, 2018 at 14:25