```ts

/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  return convertCelsiusToTemperatures(celsius, "fahrenheit");
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  return convertCelsiusToTemperatures(celsius, "kelvin");
}

function convertCelsiusToTemperatures(celsius: number, targetUnit: string): number {
  if (typeof celsius !== "number" || !Number.isFinite(celsius)) {
    throw new Error("celsius must be a finite number");
  }
  if (celsius < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }

  if (targetUnit === "fahrenheit") {
    return celsius * 1.8 + 32;
  } else if (targetUnit === "kelvin") {
    return celsius + 273.15;
  }
}
```